The exact line is this one:
「2番手にして「1億2千万」、人に従う様な男には見えないが・・・船長の度量が伺えるな」
So, first of all, it's a fact that nowhere can be read the specific word used for first mates in this story; and second, both a quick translation of it plus other versions I've checked understand it as a reference to Zoro being "second person" or "number two person" (2番手) and that's it while speaking in the context of his bounty being so high yet less than his captain's. This nibante sometimes refers a second position in a rank, as far as I recall, which isn't the same as "second in command" and certainly not "first mate" which, again, is a specific word Oda uses and is nowhere to be seen not even in Zoro's Vivre Card.