In both games I have played so far, I can change my government form on turn 1 if I wanted to. However, the tech xeno anthropology offers the perk of being able to switch government type although I already can. Is this an oversight that has somehow survived years of patching or am I missing some subtle trick?

Can't be

I could imagine your faction does have Dictatorship government, what allows you to switch the leading political parties. So my guess is, you mix up government types and leading political parties - as long as you don't make use of a mod allowing you to switch government types, without having the tech for.