Luffy got the Emperor title, he isn't scared of facing an Emperor. 2 Emperors + their crews is too much, just like how 2 Admirals is too much.
Doesn't matter if he got the emperor title. He is fairly new and as stated the world government made it sound like a group effort with the defeat of kaido which diminishes the look of luffys strength hence why greenbull thought he had a chance.
Greenbull had zero fear for kaidos crew since he didn't bat an eye fighting the two strongest individuals on kaidos crew (though they were weakened). So that's an excuse that you are trying to use.
Ontop of that why would greenbull or any admiral be scared when the have the marines that consisted of cp9, cp0, sword,, other cipher pol memebers, vice admirals, rear admirals, seastone weaponary, warlords (formerly), SSG. If the admirals had all of these things at their disposal it makes zero sense why they haven't defeated admirals by now if they are comparable to them in strength.
They aren't hence why they would not need those things on the first place