As someone with major dietary issues due to autoimmune conditions (essentially vegan + fish - tree nuts - nightshades - gluten), who was diagnosed as Asperger’s ~20 years ago and has a child with “severe” autism:

I’m both hopeful and doubtful of studies like this. I tend to think these help narrow the search space, but are missing whatever underlying issue is causing the thing they see. My sons diet is hugely restricted due to his intolerance of many things (chemo changes to taste hurt that significantly as well) so I’d imagine his gut biome is an issue at this time. It likely was NOT prior to his chemo (though we can’t know for sure) as he ate the same diet my daughter did (and we tend towards quite healthy). We’ve investigated other possible environmental factors, including PFAS/PFOS testing, heavy metals testing, etc. (my career exposed me to those but he was never exposed that we can tell). Everything inconclusive.

As I said, I’m hopeful we can figure this out. Each step helps, even if it’s not the final one to finding the cause.