I suspect you're thinking of 3.5g amount - which is a typical amount of magic mushrooms to consume. The study is using psilocybin directly.

Wikipedia states "The concentrations of psilocin and psilocybin ... [is] 0.37–1.30% (dry weight) in the whole mushroom".

Assuming about 0.5% concentration, 3.5g of dry mushrooms is about 18mg of psilocybin/psilocin.

The study used synthetic psilocybin. In some estimate I've found online (I don't know how accurate) it claims 10mg is about 1.5 grams of dried shrooms of common varieties.

Nope.