They dont.
Which is why you still have statements like i highlighted. Info about Nika was surpressed and Reward for beating Kaidou/BM split.
Luffy is considered a Yonko but at the same time not as respected as the other established Yonko are. Lucci will never look at Kaidou or Shanks and tell him "I refuse to acknowledge you as an Emperor".
This is deliberate to make sure Luffy & SHP showing off looks great narratively.
Info about nika was suppressed and this is nothing new tbh. WG is known for it.
Yet, Luffy bounty poster has G5 picture.
See, you can argue Luffy will beat kizaru but the reason you are giving isn't logical.
I mean even if you think rewards were split then as well Luffy, kid and Law defeated two yonkous in single night (that's in manga). 3 pirates defeating two yonkous is still massive deal and speaks of strength.
And, does it really matter what bonney and Lucci have to say? I mean you can always come across such characters in future who will doubt luffy so does he has to prove everytime?
And, their remarks have more to do with something else....
Lucci - Luffy defeated him. He simply in denial mode. He can't accept how SH grew so much in strength.
Bonney - fellow wg member
Lilith - because SHs were acting goofy.. nothing new. It always happen anyway.
And, I do agree that a yonkou defeating admiral not happened till now so it might be shocking incident however, it has nothing to do with world doesn't believe he is yonkou level.
Its because world haven't seen a pirate destroying naval fleet led by 30K marines + 9VAs + an Admiral...
The shock isn't due to they underestimate Luffy but because they didn't expect an admiral to go down ...
Big difference