prefacing this with IANALCUM (i am not a legit cult understanding mechanism)

I imagine it has been a cult from the start, or at least the primordial soup of factors before any of this hit the internet in earnest had all the right ingredients for a cult:

• A leader claiming to have nigh omnipotence, and some version of high charisma amongst potential followers (Yud, who despite everything, is charismatic within the confines of the ratsphere)
• A framework of lore (in this case rationalism) in which recruits are to be indoctrinated and taught how to think
• An upwards power structure in said lore that concentrates authority at the apex of the heirarchy (iq as intelligence metric, blindly doing what higher iq people say)
• Purity tests that create a positive feedback cycle that reinforces adherence to doctrine (either you believe yud about many worlds/AGI/whatever and are on the road to smartness, or you don’t and you will be cast out of the ratsphere)

I mean the list goes on. As to when it became culty? To use a trusty thought experiment, it’s the paradox of the heap.