This may be true, when "synethetic" and "psychedelic" are used as specialist jargon for describing films or various effects in film. But it's not true in the general, colloquial sense of these terms, which refer to either a type of sensation or to a type of drug.

Psychedelic literally means "mind-manifesting", while the etymology for synethesia comes from "sense" and "together".

The word "psychedelic" was specifically coined as a way to describe the drugs that were previously called "hallucinogens" and "psychotomimetics", and it was an attempt to remove the inaccurate and negative connotations that those other words had.

Synethesia, on the other hand, refers to a specific type of sensation, and not to a drug.

Also, psychedelic drugs have many, many other effects apart from synethesia (which isn't present in every trip or for everyone taking psychedelic drugs, anyway).

So in the usual sense of these terms (not in the sense a film critic might use them), these two words have quite different meanings.