no it wasn't ever confirmed that zhongli was the geo dragon sovereign and in fact i don't believe he ever was at all! i think of it as just "all sovereigns were dragons but not all dragons are sovereigns" type of deal. in regards to zhongli being a dragon, it's said in the cn version that his form is actually half-dragon/half-qilin so it's not really like he's a "full dragon". my impression of it (and excuse me for any misinfo bc i'm not actually the most knowledgeable about this part of genshin lore so my memory might be kind of fuzzy) is that the sovereign dragons are more of the vishap sort and zhongli is more of an adeptus dragon sort. neuvilette's lines about zhongli actually refer to him, and all the other archons, as "usurpers" so there's a sense of distance there in regards to their positions.

the only characters that were canonically confirmed to be sovereigns are apep and neuvilette. although there are some theories about dvalin, azdaha, and kapatcir (?) being the sovereign dragons of their respective nations but again nothing really concrete.