They probably mean this small site is caddo as in "Spiro mounds caddoan" but I don't think people have settled to whom exactly the Spiro mounds era "caddo" are really related; the old theory was they were ancestors of the modern Caddo and Witchita but it's in dispute- they may be more akin to other peoples northeast of them all the way up to Illinois. Certainly the people from the Arkansas basin are linked in some fashion- and Spiro seems to have been a trade hub or gateway in the midst of this basin where finished goods passed from places like Cahokia with its numerous copper and shell workshop sites and huge caches of chert drills in return for something- some say Spiro may have been the conduit through which flowed soft bison hair for textiles, meat, bow d'arc staves for bows, etc, since we're not sure. Spiro is lacking in workshop sites but abounds in exotic finished goods. Trade in perishable products from their region and from the west is most likely the reason but it could be that goods went to Spiro for other cultural needs- a form of tribute, a kind of religious sacrifice at what may have been considered a special site, or reasons we can't understand yet.
The time I've spent at Spiro itself was somewhat of a waste, as I didn't really know a lot of "big picture" background as to what I was learning until much later, when I got into my own genealogy.
I am Chickasaw and do not consider myself related to Caddo or Wichita, as we were Muskogean in language family. Our historic homeland is around the Tennessee River in NW Alabama and NE Mississippi.
I consider us part of the "Southern Cult" or Mississippian culture - however, some say we came from west of the Mississippi in the ancient, and probably originated around the Red River valley, like the "Caddoan" cultures, before migrating eastward.
One thing, though, we were not mound builders and there is nothing in our anasazi origin stories indicating that we ever were. That's why I am so often in dispute about that.
After Removal, we were originally part of the Choctaw Nation, then "got our own place" - but we own half of the Arkansas River. I believe Spavinaw is in the Arkansas watershed, although it might be the Grand River - I'd have to look.
Anyway, I dislike that these ancient artifacts would just be handed over to the Caddo-Wichita tribes, no questions asked. I looked at the Caddo Nation website just now, at least the one sponsored by their Texas portion, and could not believe the hyperbole they've written.
You would think they are the only Indian tribe in the US and that they invented everything and saved the white man from every other evil Indian - and made the most fabulous pottery in the world! They are laying claim to everything that has ever been labeled "Caddoan" as their own modern-day tribe's birthright! Harrumph.