There isn't much to paraphrase, really. When my doctor was prescribing fluoxetine he offhandedly mentioned that he would start me off with a lower dose than he normally would for a person of my size and issues because I was already taking amphetamines, which meant that I wouldn't need as high a dose. The offhanded way he mentioned it gave me the impression that it was common knowledge and later another doctor also mentioned the same thing to me, but this one was not a psychiatrist but a cardiologist, farther cementing my suspicions.

When I later asked my psychiatrist about this he said that since SSRIs and amphetamines are though to complement each other (even if the science on the matter isn't really decided) the recommendation they get is to adjust the dose of SSRIs down. I'm sure the actual reason is more complex than that, maybe involving fears of serotonin syndrome, but my takeaway from it was that the effect is well known.

No idea about coffee in particular since I wasn't allowed to have any caffeine during that time.