This is misconception. It's not about the extremism of the belief. It's about the mechanism of control.
Take the "cults" of the moment: Qanon/MAGA, for example, operate like a personality cult in many identifiable ways, but are not actually cults, because you can leave without much difficulty if you set your mind to it.
On the fringes US politics encourages ostracising family members who do not agree, but these movements do not have a mechanism of control (financial control combined with a commitment to cutting off family members).
There may be cult sects within MAGA/QAnon that get close, perhaps (just as there are at the fringes of all strange and mainstream religious beliefs)