> I think this is the clinical trial registration: https://clinicaltrials.gov/study/NCT03408886?tab=results

That's a slightly different clinical trial on adults, not children like the posted article. I think this is the clinical trial registration: https://clinicaltrials.gov/study/NCT02504554. Here's the followup report in 2019: https://www.nature.com/articles/s41598-019-42183-0

That said, your reasoning is probably still correct. There's no placebo group, and a lot can change over a 2-year follow-up, with participants aged 7-17. Maybe they went to speech therapy or just matured and learned more coping behaviors. The 2019 followup also notes that 12 of the 18 participants made other diet and medication adjustments. They claim the adjustments were minor but that's still more noise, and it doesn't account for unreported social/environmental changes.

> There have been so many small scale trials showing amazing autism improvements that failed to replicate in larger, better controlled trials. I wouldn’t get excited yet.

Unfortunately yeah, it's unlikely anything exciting will replicate in a larger RCT other than maybe the gut biome improvement since that seems directly mechanistic, but that's just a gut feeling.