> The more we build the body of research on top of obviously flawed studies like this, the harder it is for serious researchers and lawmakers to take it seriously.

I agree that what should be called "research" should adhere to the highest standards and actively seek to avoid biases.

But generally (not specifically about this psychedelics study), it's OK for us to act upon imperfect, biased information. The absence of a rigorous proof that something works, does not mean it doesn't work. The presence of an imperfect proof is enough to increase the probability of a positive outcome.