you don't remember why, do you?

If you can predict someone's coffee intake based on testing of their microbiome then you've proven that coffee intake has predictable effects on the microbiome.

The important part isn't predicting coffee use, it's just the proof that there's you can predict and perhaps control in the opposite direction leading to more research.

One fun fact: we still haven’t figured out why coffee makes us poop. We’ve studied every chemical in there and can’t seem to find a link, but the association is uh… well-known.

https://rarehistoricalphotos.com/nasa-spiders-drugs-experime...

But they did test both caffeinated and uncaffeinated coffee, and found the same effects in both, indicating that the effect is caused by something in coffee other than the caffeine