From that link, it seems like the change in the text column width behavior, which is promoted as limiting text columns to a maximum width, in practice creates a min-width as well which can be quite wide. That is, in the old wikipedia, you can adjust the browser window to make the text column arbitrarily narrow, but in the new wikipedia — depending on other elements on the screen — in practice can you from having an uncomfortably wide column of text that you can't adjust to be any narrower.
So it seems like kind of a mixed bag readability-wise.
edit: For example, this article's main text area seems to have a width of around 155 characters, while typography rules of thumb dictate not exceeding 75 or so:
There could be other factors involved, but it takes longer for fr.wikipedia.org to load on my computer than en.wikipedia.org. I hope Wikipedia will remain usable on old and slow devices.
So it seems like kind of a mixed bag readability-wise.
edit: For example, this article's main text area seems to have a width of around 155 characters, while typography rules of thumb dictate not exceeding 75 or so:
https://fr.wikipedia.org/wiki/Reichsgau_Wartheland