I have just built a pair of wheels based on MTB "29ner" rims but I don't think they would deal anything less than 28s.(which I am running on that set)
I did fit a 23 just to see and it looked way to small/sketchy.
I believe it is part of a quid-pro-quo deal where Vélib's parent company get control of outdoor advertising space in Paris.
Vélib can't run at a profit.
Or just spit on it. (not my wee wee)