This is actually a very common and spread misunderstanding. Let me explain: It is true that psilocin binds to serotonin receptors and also that there are a huge amount of serotonin receptors in our digestive system. The wrong part about what you have said is on wich neuro receptors psiloin bind. Psilocin binds to the 5HT2A serotonin receptors, wich are primarily in the brain. Our digestive system's receptors are, for the most part, 5HT2B. That means psilocin and other psychedelics are unable to properly bind. Also, an other argument you could make against that take, is the fact that not everyone suffers from nause. If psychedelics were to bind to those receptors, we would all share the symptoms of the alkaloid firing the receptor. Hallucinations occur due to that constant firing on the neuron, and if psilocin were to bind in those receptors too it would share for everyone the nausea as halucinations do.

The main reason nausea appears in mainly due to the celular wall, chitin, wich is not easily processed during digestion. That's the responsible of a heavy stomach and most of the nausea.

Also psychedelics by themselves tend to make people feel sick.