And what is that theory based-on?

Psychedelic research has been slow for years, and it's partly due to reasons such as people believing that a mushroom is a mechanism for communicating with intelligent life throughout the cosmos. I'm not saying the theory is wrong (though I'm pretty certain it's wrong) - but given that we have zero proof of intelligent life outside of Earth, and that the speed of light is a limiting factor in the speed communication (ignoring entanglement as a potential form of communication for now), how has it been extrapolated that a mushroom could be the solution to these limits?

It's likely the theory has come from a user of psychedelics, and one thing that frequent users of psychedelics fail to understand at the time (this is from experience) is that reality is pretty slippery. I would estimate that in a single year (a few years ago) I experimented with N,N-Dimethyltryptamine 200+ times and that's just the kind of theory that I'd come up with at the time.

Aliens told me during a trip, and then I found out others had the same experience. If nothing else it's pretty interesting psychologically.

It's definitely interesting psychologically, especially N,N-DMT, considering many of the users all share many of the same trip experiences[0]

[0]: https://www.vice.com/en_uk/article/5gkkpd/dmt-you-cannot-ima...