I think Green Bull is not properly revealed for that reason.
Going forward i expect Sanji's opponents to be tech users/modified humans, and i think that's going to be the case with Green Bull.
That's going to explain how he went without food for three years.
Also Green Bull talking about pretty girls feeding him, that's the type of personality Oda would write for the Admiral that Sanji would go against
Also it looks like he might have cooked for Fujitora, with Oda focusing on Fujitora thanking him for the food and praising it twice. So he might be quite the skilled chef even though he does not eat the food he is cookin.
And he asks Fujitora "How was old man Vegapunk", which could imply he's an old acquintance of his, and that would make sense if he took part in Vegapunk's experiments