I don't think he's there with the intention to fight, but if he were to be portrayed as a legitimate threat to Luffy and the others who are YC1 level or above, then yes, he's stronger.
Even if you told me that Luffy couldn't beat Greenbull on his own, what's stopping them all from doing a 6v1 on his ass and annihilating him? Greenbull would've come with the SSG, whatever the hell they are.
So either Greenbull isn't looking for a fight, or he's much stronger than a Yonko. I prefer the first option.