Bottom line is there is no way for any individual to really know if they have latent schizophrenia anyway, and I think it’s a conveniently non-disprovable explanation for why psychedelics and cannabis aren’t bad.
In this specific case, the authors started him on a treatment for schizophrenia and he got better. Why continue him on this treatment if they thought it was simply the cannabis that caused it (as he would abstain from then on). Also, they increased his dose prior to discharge.
I'm not arguing that cannabis didn't cause his schizophrenia, just that this doesn't sound like cannabis induced psychosis, which is a different diagnosis with different prognosis.
I do think some people can have that "latent" potential yet have the good fortune of never activating it – and cannabis definitely has the potential to do so (although most recreational drugs probably can given sufficiently heavy long-term use).
(I tried cannabis once, years ago, I could see why people like it but I didn't feel any great urge to repeat the experience – and worries like this also discourage me – which is probably for the best.)