by illegal, I believe the poster was mostly referring to child or violent content as opposed to copyright.

My understanding is that they and their subsidiaries actually own a lot of the content that you're probably thinking is "illegal" or "copyright infringing" [1]

[1] https://en.wikipedia.org/wiki/MindGeek

I've seen a lot of like "boutique" porn videos on PornHub that seem to have come from small-ish porn sites. Don't really know if PornHub owns the rights to those. Also, I've also seen some foreign porn flicks there and I also had my doubts about those.