The thing is, anecdotes can be easily made up. I’m not saying that’s happening in the case of visual snow, but we need studies to talk about things like this, not anecdotes.
Except of course we can't know for sure the anecdote was an example of LSD causing visual snow, the drug might have been spiked or it could've just been a coincidence...
Anecdotes are at best a clue that there's more research needed. Unfortunately for many people they're also often far more memorable and even convincing than cold hard statistics.
And if someone tried to argue that LSD can never cause visual snow, a single anecdote is enough to refute that. Anecdotes are not always wrong in debate.