Bartolomeo's words aren't Oda's words; he explicitly adds a "(BY ME)" to his consideration of Zoro as the first mate. It isn't an official, objective title in the slightest, Bartolomeo isn't saying that Zoro is the first mate but that he personally considers him as such (on the contrary Oda wouldn't need to specify that "(BY ME)"). Anyways, I wouldn't be too confident on Bartolomeo as a source of arguments because he's a caricature of the fandom, and I'm precisely discussing how this very fandom unfoundedly gives the title to Zoro.