I'm sure that our bodies produce molecules that interact with the same receptors that psilocybin does. But that doesn't mean that our bodies produce psilocybin.

Psilocin is 4-hydroxy-DMT, and psilocybin is just phosphorylated psilocin. DMT is itself just dimethyl tryptamine, which is a modified serotonin. Serotonin itself is itself just modified tryptophan (without the hydroxyl group). The body produces small amounts of DMT naturally, I wouldn't be surprised if that is also the case for Psilocybin. Biochemistry is messy and small amounts of many "unintended" products are created.

Of course... serotonin.

can you elaborate on that?

I'm guessing it's the production of DMT (similar to psilocybin and LSD) by the pineal gland that is being referred to. IIRC there's still a debate about whether it can cross the blood-brain barrier, but it seems quite likely. Ostensibly it's produced at birth and at death.

Please excuse the lack of sources, but the book The Spirit Molecule kickstarted this line of thought if you're interested in learning more.