No, that's not true. I have given some justification. You could fairly criticize my justification as inadequate, but not as entirely absent.
And you're right. I would not claim to have given adequate justification for my assertion. I can't prove that Urbit is a cult. It's just my personal opinion. If you don't like it you can return it for a full refund.
But I will say this: my opinion is informed by some pretty extensive knowledge of the technical parts of Urbit, especially its core design, and personal first-hand knowledge of the history and motivation behind its design. Again, I can't prove any of this. I could not publish it as a peer-reviewed paper. All I can do it wave the warning flag. How you choose to act on that is up to you.
Typically the test for a cult is:
1. Secret knowledge
2. Enforced isolation
3. Some kind of leader/living authority
Eg, Scientology has their secret books, they will attack you and sue you if you share that secret and they have church elders.
The founder of Urbit, Curtus Yarvin, is an extreme hard-core right-wing liberatarian. Go read up about him:
https://en.wikipedia.org/wiki/Curtis_Yarvin
https://en.wikipedia.org/wiki/Dark_Enlightenment
I'm not even sure that Curtis would object to my characterization of Urbit as a cult. He and I are not exactly on speaking terms any more, as you might imagine, but he is just enough of an iconoclast that he might actually agree with me on this.