Unfortunately I think the issue is a little more generic than that. If I'm reading this right, it'll set the culture level of the city to the "correct" level, but there are other parts of the code that do checks like "Is culture level of this city the same as the number of culture infos? If so, then it counts toward culture victory!" and such.
It does work as a spot fix for the border issue, but there's a number of places that'll need similar spot fixing, either already or as a result of this fix.
Could be done this way with spot fixes, but I think (?) updating the getNumCultureLevelInfos and getCultureLevelInfo functions together might be a better solution? Or not, for some reason I do not know.
Edit: Well, actually there might be a good reason in that I think those two functions are wrapped up in savedata... somehow, the wrapper read/write is still a mystery to me, so maybe spot fixes, though ugly (?), could be the way to go if it's being done without savebreak. The actual spotfixes seem to be, at a quick search:
5+ places related to cultural victory condition, the mess with CultureRangeCities in a few places, some AI evaluation, Forgetful.py (?), and at least one event.
Granted these haven't been noticed as being an issue before, but adjusting the threshold in this place only will have knock-on effects on most of these, and so at least the important ones will need to be fixed (i.e. culture victory conditions).