It's technically possible that early-life microbiome differences from bypassing the birth canal could influence it, but the fact that the association disappears when siblings who were delivered differently are compared is suggestive.

"However, the association did not persist when using sibling controls, implying that this association is due to familial confounding by genetic and/or environmental factors." - https://jamanetwork.com/journals/jamapsychiatry/fullarticle/...

That’s interesting but it’s hard to conclude anything without measuring titers during births which do and don’t result in ASD.