No one is forced to defend shit, the guy has an amazing showing. Don't mistake this toxic bubble of retards power scaling lip quivers for the majority of rational readers who would look at you people as mentally insane or trolls who wouldn't be worth the time of day. An intelligent reader doesn't need any more evidence beyond Oda himself on multiple occasions placing the Naval Admirals and Yonkos in a similar bracket as the two factions who sit on top of the power hierarchy to come to the conclusion that a single Admiral would be a threat to a single Yonko and vice versa. But now we have to deal with people who are incapable of differentiating power portrayal to character personality and narrative decisions; Greenbull being clearly an emphatic, over the top, animated character is as obvious as Queen being the same, hence why his reaction should be taken into context and understood in accordance to his dialogue.
Greenbull is just as restricted by the narrative as Fuji was in Dressrosa, it's just not as obvious. Greenbull is ultimately only here to be officially introduced and more importantly prove two things, one is that Wano is safe guarded and doesn't need outside protection and two being that the straw hats have reached a level where an Admiral arriving won't have them tripping all over themselves anymore. There's also the necessary growth of Momo into a brave and capable leader like his father which will take precedence over Greenbull swinging his dick around and showing why he's "the boss", because the former was an integral part of this arcs story while the latter is just a power scalers wet dream. If that isn't enough for you then just take it from the man himself that is telling us he's ready to fight Shanks and his crew just not yet, which makes sense since the man would need some backup before going to war with a Yonko, seems pretty reasonable unless you expected something of him which even prime Roger would be incapable of doing solo.
At the end of the day Greenbull was never going to show his full arsenal since he is undoubtably a future opponent and that is enough of an excuse for his "lack of feats", if wiping out two top commanders with ease then going on to subdue multiple commander level threats while having four monsters on guard ready to jump you at any moment all the while you are self admittedly not even being serious is a bad showing. But by all means go ahead and ignore everything even something as basic as how a Logia works for the sake of your agendas.