I don't disagree that these substances are known triggers for those with predispositions, but be careful with your logic - that's exactly the same format as autism/vax conspiracy folks use, when in reality the common cause is age. Symptoms of autism can start to manifest around the time childhood vaccines are being given, just like schizophrenia and related conditions manifest in teenagers and young adults (also groups known to try drugs). In the latter case we do also have evidence that psychedelics are associated with increased risk/earlier onset, but you wouldn't be able to tell from the timing anecdote.

How do you know that it's not the other way around - they started using hallucinogens because their schizophrenia started and the confusion around a drastic change in mental state caused drug seeking behavior?

Schizophrenia commonly starts showing symptoms in the mid to late twenties - so while the two may be linked it's also likely that schizophrenia onset happens to be the age that many people start seriously taking drugs.