So I think that a good rational materialist skeptic, even after repeated hefty doses of the chemical resulting in repeated interrelated and seemingly impossible experiences of contact with vast inhuman intelligences, would have to stop short of concluding that DMT is an actual gateway to contact with whatever sort of alien the individual experiencer feels him/herself to be in contact with.
Instead the skeptic can settle somewhere in the neighborhood of a pleasant bullet list of possibilities, some of which could be eliminated with proper experimentation... most of which are not exclusive to one another...
* Everyone doing this is making it up ( very easy to eliminate this as a possibility, if it is untrue, if you have access to the chemical )
* People hearing highly detailed accounts of alien contact from others subconsciously program themselves to have a similar experience.
* The human imagination is capable of supporting apparently omniscientient telepathic lifeforms under the influence of extremely powerful psychedelic drugs. An extension of the childhood capacity for apparently autonomous and self-determining "Imaginary Friends".
* Aliens created humanity and put DMT all over the place in grass and trees and roots and frogs and fish and human brains as a method for humans to contact said progenitor aliens. (EXTREMELY DIFFICULT TO FALSIFY THIS ONE)
The list can be really long! But people getting into this stuff can afford to get skeptical about the specifics.