It was my understanding that upon the ingestion of mushrooms, psilocybin gets dephosphorylated to psilocin, which then acts on the serotonin receptors, and none of the effects were due to psilocybin binding to them directly.

However, this article: Binding of Psilocin and Psilocybin to Serotonin Receptors says psilocybin itself binds to serotonin receptors, although will less affinity.

Does this mean the in vivo dephosphorylation is not complete, and could the difference between the potency of plain mushrooms vs mushrooms prepared using the lemon tek be due to the difference between the affinities of psilocybin and psilocin?

The two issues are not connected; psilocybin could bind to 5-HT receptors in a scientific experiment setup but still be completely dephosphorylated to psilocin in a human body before it ever reaches any brain receptors responsible for producing psychedelic effects, or it might not be able to pass through the blood-brain barrier well at all due to the phosphate group. Most evidence I've seen suggests that this is the case, but if psilocybin actually does manage to get into the brain in any significant concentrations somehow, then its 5-HT receptor affinities might make a difference to the psychedelic effects.