The article extrapolates from very scant evidence that there was little to no prejudice against people with the condition, across the breadth of the middle ages, with little evidence for cultural differences. This seems to be incredibly poor history. What do we know about Down's Syndrome in 6th to 16th century Europe, the Middle East, and North Africa? What is the earliest attestation to symptoms characteristic of Down's Syndrome?
I think you're being unfair to the authors of the article. They only say that in this case funerary practice suggests the child wasn't treated differently, not that this can be generalized for all of the Middle Ages or even the Early Middle Ages.
The abstract of the article also answers your second question:
The Saint-Jean-des-Vignes child is the most ancient well-documented case of DS.
Unfortunately I don't have access to the article itself so I don't want to give a definitive statement on this but generally most early medieval archaeologists would frown upon using funerary practices such as this as a strong indicator of social value, though this depends of what school of thought you follow.
The school I was brought up in and the one that makes most sense to me regards early medieval burial with grave goods as much a social performance of wealth and power for the survivors as an expression of grief. So this burial could just as well be interpreted as a family showing that they have the wealth to bury even their most shunned members in state.
I don't mean to derail the thread and I hope a medievalist answers OP's question, but I think this is also an interesting methodological question. What other types of evidence would a historian need to make this claim? How do historians use archaeological evidence of this kind? If the archaeologist makes a claim like this, are there professional expectations for them to use historical evidence to back it up? Further, in a history of medicine seminar I did we all had enormous problems tracking down historical record of disease, for reasons of different language and vocabulary, as well as the problems of reconciling modern pathology with medieval ideas about bodies and disease (humours etc.) Essentially, how would one even go about writing a history of medieval Down's Syndrome? As a 20th c person it seems impossibly complicated.
This only refers to the specific instance of Down's Syndrome if I understand it correctly. So it doesn't rule out that the condition was known to earlier peoples.
Did Galen or other early physicians know of the disorder, and if so what did they have to say about it? If not Galen, were there others?