It’s important to note: the study only had 20 participants, so it wouldn’t have enough statistical power to detect a moderately large effect. Failure to find an effect is different from evidence there is no effect.
This is a within-subject experiment: each participant got all 4 possible doses. The paper (https://www.gwern.net/docs/nootropics/2019-bershad.pdf) doesn't discuss power that I can see, but the power of a within-subject design will be a lot higher than the between-subject you're probably thinking of, where n=20 is generally way too little. So, might be decently powered after all.
Also, why did they not go for a dose every 3-5 days like the anecdotes, but instead of went for every 7 days. And then say it might be different when done every few days. Were they trying to avoid testing on the weekends or something?
That said: I know firsthand that people who are borderline schizophrenic can have a very bad, long-lasting outcome from a regular dose of LSD. I doubt a microdose would do them any good. I hope that in studies like this, they screen the participants carefully.