Given there are cases of sudden onset autism being resolved with antifungals, it’s at least not implausible that fecal transplants could be effective too.
They’re always followed by thousands of self-experimenters where nobody can reproduce the result.
The explanation is almost always placebo effect. A parent or doctor is so convinced that they’ve found a cure that they change assume it worked, change their behavior toward the autistic person, and believe that a dramatic change has occurred.
This is also why the placebo arm of every autism study also shows improvement.