The diapsid part is very likely indeed, as fossil skulls of early stem turtles do show some temporal openings ( annualreviews.org/…/annurev-ecolsys-110218-024746 ) The point is more where do they nest within Diapsida, more closely to the Lepidosauromorpha, or to the Archosauromorpha, and where precisely if within one of those clades. The point is that can’t quite be proven using only extant species, whether by DNA or morphological evidence. And concerning ML, the methodology is often criticised, not because it’s bad, but because it’s opaque and thus it is difficult to justify and understand as a process