To clarify, as far as I understand, "set and setting" matters when taking psilocybin to get high, but microdosing is taking psilocybin (or other similar drugs) to very deliberatly not get high.
So what does "set and setting" have to do with microdosing, and why doesn't it matter with aspirin, which also doesn't get you high?
Now I've never microdosed mushrooms, but it's not hard for me to imagine that this same concept applies, of actively pursuing a good experience, which can steer that experience in a good direction. It will just be at a much smaller scale.
For myself I think of this phenomenon as a kind of self-fulfilling prophecy (not meant as a judgement about others) which helps me to keep in mind that I'm the one in control (assuming doses are not excessively high, I think others in this thread call those "heroic doses").