You can always test out this hypothesis by doing a tab of LSD from the same batch twice while controlling for your headspace. One day, you do it with a positive headspace, and another day you do it with a negative one. If you had a great experience the first time and a bad experience the second time, clearly the headspace alignment made the difference.
Yes, this kind of testing isn't as rigorous as it would have been for a legitimate scientific trial (which I bet we will see in the future as the research into the area accelerates), but imo it is more than just a No True Scotsman backed by broscience.
And of course, no one says that a negative headspace is the only reason for someone to have a bad experience, there are others for sure. For example, certain people with predisposition for schizophrenia probably should abstain as well. AFAIK psychedelics won't cause a schizophrenia in an otherwise healthy person, but I heard it might kick off an early onset if you were going to get a schizophrenia in a few years without any psychedelics anyways.