Is Quidditch considered a sport? Are questions about Quidditch permitted here? Although some countries have established rules for Quidditch games, I am unsure whether or not it would be considered a sport on Sports SE?
There's obviously a line somewhere at which a sport is so niche that questions on it are going to be of no value to the community (e.g. "this game I made up yesterday"), but Quidditch has (in my never humble opinion) enough of a following and players that it's on-topic here. Personally, I think it's a bit silly, but that's neither here nor there.
For avoidance of doubt, I'd very strongly say only questions about the "real world" Quidditch (as you've linked to above) are on-topic. Any questions about the "Potterverse" Quidditch (with people flying around on brooms, etc) are off-topic here; they may be on-topic at Science Fiction and Fantasy.