So if Yes does win, is there any argunent the anti-statehood camp could put forward to Congress on why the referendum is illegitimate?
Yes, quite easily. See the CNN article above. But i think more importantly - Velasquez and AOC just came out with their own proposal - essentially MVC's. Hard to see statehood happening without Velasquez in favor.
Not really seeing that in the article. A lot of arguments for why people should vote No on statehood, but I don't see any arguments for why Congress shouldn't respect a Yes vote (other than Republicans getting in the way).