I was always under the impression that the legal jurisdiction phrase was meant to exclude foreign nationals such as ambassadors and staff who reside here for diplomatic purposes but are under the jurisdiction of their home country.
In Wong Kim Ark the SCOTUS simply decided that the Chinese exclusion act could not be retroactively applied to someone who acquired citizenship by reason of birth within the United States, which was the law at the time of his birth.
In other words, the 14th Amendment does not require birthright citizenship. The Congress could act to exclude such cases if the corrupt, lying pieces of dung ever acted to do so. Indians and Puerto Ricans were not considered to have US citizenship until Congress passed a law granting them such, even though they were clearly "under the jurisdiction of the United States" in the ordinary sense.