The article only starts from 1944, as if the creation of these countries started from then only. It conveniently omits the fact that even Yugoslavia (means Southern Slav) was a continuation of the Kingdom of Serbs, Croats, and Slovenes, which was established after WWI. Kosovo (and part of Macedonia) was part of the new country. Since the new country is a merge of three kingdoms (Serb, Croatia, and Slovenia), we can tell that Kosovo was considered part of the Serbian Kingdom (as Croatia and Slovenia, being both Catholics, never lay claim on it).
The Western assumption that Serbia enjoys sovereign rights over Kosova, however, is as fictitious as the Serbian myth that Kosova was the cradle of the medieval Serbian state...
Historically, since during the Crusades, Kosovo has been part of Serbian Kingdom for long time.
Culturally, Raska, or Kosovo, region was the place where King Stefan Nemanja (1167-96) reigned. He was the first to proclaim Serbian independence from the Kingdom of Bulgaria, a strong country in the region at the time, and also include Zeta, the other (and older) Serbian-ruled area. Stefan later went to Greece, entering Hilandar Monastery on Mt. Athos. Also important to remember that Stefan's son, prince Rastko Nemanjic, who in young age already decided to become a priest, later became the first Serbian Archbishop, which practically establishing an independent Serbian Orthodox Church. He later became St. Sava, whom until today is considered the Patron of Serbians. The biggest church in Belgrade, St. Sava, is after his name. For this reason, Serbians consider Kosovo the cradle of their civilization.
When the Ottoman invaded the whole region, Serbian fought hard and the biggest battle of all was in fact, in Kosovo, where they lost their prince, Lazar. One by one, the Christian areas in Balkan (at first mostly Orthodox areas) fell to Islamic Ottoman, and by the 16th century, in included Catholic areas of Croatia, Slovenia and Hungary. During this period, migration of the Muslim Albanian to Kosovo, together with conversion of some local Serbs, changed the demographic-religious structure of the region.
By 18th century, the Ottoman started to lose some areas in the region to Hapsburg Empire, including Croatia and Slovenia. In early 19th century, some Serbian rebellions led the Ottoman to grant them an autonomy status, while other parts, such as Bosnia and Kosovo, were still under Ottoman.
After the Second Balkan War, 1913, however, Serbian acquired the Kosovo area, and when the Kingdom of Serbs, Croats, and Slovenes was established in 1918, Kosovo was included. The new Kingdom gained international recognition under the Versailles Treaty (1919).
This new kingdom, however, was always troubled by internal conflicts, many times because leaders from non-Serbs parts were dissatisfied with Serb-dominated government. Before and during WWII, the conflicts were stronger as the leaders accusing each other of collaborating with the German. Tito brought them together and established communist federation, Yugoslavia, with Vojvodina and Kosovo as autonomous provinces. This part is being used by the article’s author to claim that Serbs doesn’t have any claim whatsoever of Kosovo.
So, I see this article as a piece to confuse readers with claim as if Serbs’ claim over Kosovo is totally unfounded. It's true that the issue is not easy to resolve now, as majority of Kosovo's population today are (Albanian) Muslims. But, to say Serbs’ view of Kosovo as their cradle of civilization is a myth, I think, is a myth.
If it had chosen to go further, it could also have explained why Kosovo was never legally part of contemporary Serbia until 1945: the treaties of London (1913) and Istanbul (1914), by which the Ottoman empire ceded it to the Kingdom of Serbia, were never ratified.
The legal status of Kosovo in the Kingdom of the Serbs, the Croats and Slovenes (SHS), called "Yugoslavia" after 1929, is even also in question. The Treaty of Sèvres, signed by the SHS Kingdom with Turkey in 1920, became null and void, and the Treaty of Ankara of 1925, which involved the mutual recognition of the states, made no specific mention of the territories taken from the one the other in 1912-13. You can only say that the recognition of such annexations took place, and only implicitely, when Ankara opened a consulate in Skopje.
Also, the second Yugoslavia was expressedly built on a rejection of the institutional principles of the first, which had failed because it was a centralized state according to the Serbian political tradition. However credible under Communist rule, the Yugoslav idea was conceived of in Croatia in the 19th century as a voluntary union of equal partners, not as an empire where some conquered peoples were subject to recurring policies of extermination.
You wrote: for this reason, Serbians consider Kosovo the cradle of their civilization. But you gave no reason at all, and there is none: Raska, as I have said, was what is now known as the Sandjak, not Kosovo; the claim by some Serbs that their medieval state was born in Kosovo is nothing but a lie. The Serbs started invading Kosovo, at the expense of the Byzantine empire, after the Nemanjid state had become independent, and the conquest actually took place in the13th century. Is the above slogan supposed to mean that, by their own admission, they had no civilization before?
And then, in 1459, every Serbian state disappeared into the Ottoman empire. The Serbs may be the only nation in the world who have managed to convince otherwise sensible people that a 250-year possession by successive medieval states half a millenium and half a century ago is a valid basis for a contemporary territorial claim.
The truth of the matter is that the Serbian state invaded Kosovo in 1912 — and massacred 20,000 of its Albanian natives — after it had liberated itself from the Ottomans, because it wanted an access to the sea through the Drin valley (also, because it would help them hold Macedonia). All the Serbian myths now peddled about Kosovo were then fairly recent, as they are re-writings of history from the 19th century, including the claim that the first Battle of Kosovo was of decisive strategic importance.
Among these, the claim that Albanians "immigrated" into Kosovo is a formal absurdity. Since the Albanians are the descendants of the Illyrians, the first known inhabitants of the former Yugoslavia (except Slovenia), the Kosovar Albanians could not, logically, "immigrate" into Kosovo, all the less so when the whole region was part of a single state. They became an absolute majority again in their own country in the mid-19th century, when enough Serbs had left for New Serbia, the principality in the north which offered better prospects as it was de facto independent from the Ottoman empire.
The only instance of a massive influx of Albanians into Kosovo which actually took place is the one the Serbs won't tell you about: when 100,000 Albanians who lived in the region of Nis, Pirot, Leskovac and Vranje were ethnically cleansed by the Serbs in the winter of 1878. All the other stories about Albanians "immigrating" into Kosovo are Serb fabrications.