I think the appropriate response here is some combination of Russell's Teapot and Occam's Razor: they might, but some kind of hard evidence is necessary before humouring that theory, particularly since there's a vastly simpler explanation on hand.

Both alcohol and marijuana have direct actions on the brain. That’s why people consume them.

They do not exert their primary effects via microbiome modulation. This is obvious because the effects occur nearly immediately upon consumption, whereas microbiome change from what you consume is a gradual process.

The question I have is: Why has microbiome become the explanation for everything? What would lead you to believe that microbiome would be the explanation for this, when the direct action upon the brain is so much more direct and obvious? Microbiome is an interesting area of research but how did we get to this point where some are ignoring the obvious and trying to construct alternative microbiome based explanations for things like alcohol and marijuana impairing memory?

Or by affecting the kidneys, or by affecting the enteric nervous system, or through some other pathway for which we have no substantial evidence of influencing memory (yet). It just seems like a baseless prioritization of a hypothesis. For some reason, people are specifically fascinated by the gut–brain axis.