I'm no expert, but isn't paranoia one of the most significant, if not the primary, characteristic of psychosis? If cannabis triggers one, I'd say the 'strong' link is well warranted.
I've seen it many a time and never seen someone have a psychotic episode.
While they may have had some paranoia not a single one of them suffered from psychosis. On the contrary most seem quite happy, laid back and well adjusted. Most have the same issues we all have, weed or no weed.