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.
Ive had several friends become paranoid while smoking pot. Some of them have been daily for years.
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.
I've seen it many a time and never seen someone have a psychotic episode.