Technically They Weren't Satanists. SPOILERS!
Satanists basically swear fealty to Satan, not so with these folks in "Hereditary" They were after some obscure "Prince of Hell" Why? Who knows. If you see no problem with that, then you don't understand who Satan really is.
He has no friends, no allies, no confederates as it were. He's the king of pride & ego, the father of lies. He's not going to take kindly to us crazy mortals swearing love & devotion to one of his underlings. All of hell is under his domain, & everyone, Paimon & whoever else answers to him. The cult f*%d up by throwing no love his way, you just don't do that to the chief, the big cheese. His house, his rules.
So jealous, hateful S.O.B that he is, he's not given his "blessing" to some crazy cult after one of his minions. That's something this movie gets wrong. I'm surprised he didn't take hellish vengeance on all involved..
The cult can't be considered "Satanists" as Satan wasn't who they worshipped.