Opinions. It's not a cult he's a person sharing his point of view, there's not toxicity in that.
There's no harm on what he does in the same way that FFVIII was never harmed by the theories that surrounded it, even if Nojima deconfirmed them. If there's a plothole o lose end, let the fans have fun filling the gaps.
Saying that the cancelled alt timeline DLC was the "real" ending or that Eos is a goddess of the Dawn in FFXV are "opinions" is not only based on flawed and poorly formed reasoning that goes against what the game actual has established as canon and facts, but to also say there is no toxicity in that is also false since people desperate to attack XV will use anything they can including stuff from Pez's misinformed videos and "theories" to try and use against the game. Especially those still trying to prop up their precious versus crap.
FF8's fan theories were not predicated on the basis that it had its "REAL!?!" ending added years after release or on things that aren't originated from the game itself, the whole R=U thing started from things only within the game itself. Pez just thinks because the name Eos in FFXV, which only the name came from the goddess from Greek mythos of the same name who is the Goddess of the Dawn there, he thinks that just because of that it somehow means there is an actual goddess of the dawn called Eos in FFXV and he keeps using the image of the Oracle from the prophecy as if that was Eos, and he thinks that this supposed Goddess called Eos in XV replaced Etro too. Like, you can have a fanfic, but you can also be wrong and not be an idiot spreading blatantly wrong information for other people to regurgitate, because of stuff he says his otherwise uninformed viewers take his words at face value because there isn't any other Youtuber around to say otherwise and it makes having any actual discourse surrounding the game borderline impossible because people keep spreading misinformation.
Saying something is just an opinion is not a defense against being wrong or for spreading misinformation.
Like if someone went around for 2 years pushing their "opinion" that omg Noctis is really Ardyn in every single scene in the entire game because he can transform! and keeps harping on about it despite being proven wrong multiple times, and makes monetized videos with clickbait titles about the game with blatant lies, at that point what's even the point of being truthful or correctly informed about anything? Why not just lie about everything because clearly there's people willing to just get lied to non stop?
My problem with him isn't that he has an opinion, my problem with him is that he spreads misinformation and people defend him for spreading misinformation.