I'm sort of surprised they didn't extract/synthesize psilocybin directly and administer that. Decades ago, I grew a few batches of P. Cubensis. Even amongst mushrooms from the same rice cake, the dosage wildly varies. You could have a "trip" from a tiny mushroom or eat a handful of duds.
But this of course introduces more uncertainty, as you pointed out, so to have any conclusive results a much larger sample group would be needed, I think.
Also I agree with others that comparison with the baseline for each test subject, not just between the test group and control group, would be important.