It’s an asspull from Kaido for simple reasons:
It was established that you cannot train your Conquerors Haki. And that you only get stronger with it and more effecient with it through your personal growth in the other area (ie... your armament and observation)...
We can clearly see that Luffy applied the same concept of flow from his Ryuu training in order to open the door for Conquerors haki.
Him accessing that area was facilitated by his mastery over Ryuu
and his observation haki is already among the top... meaning Luffy currently already having advanced version of all 3 haki types
thus it’s no surprise for him to imbue his attacks with conquerors haki, cause his growth in haki is through the roof...
kaido on the other hand, showed ZERO signs of observation Haki
Showed Zero sign that he knows Ryuu, in fact he was surprised that Scabbards could hurt him and didn’t know how their haki can do it...
he’s clueless about usage of flow/Ryuu
yet somehow the haki which cannot be trained or elevated unless you elevate the other two, he’s somehow asspulling his way to use Conquerors haki in advanced version
at least Luffy’s power ups are logical and follows the story logic, while Kaido’s powers are just everywhere in every place, when Oda wants him to have an ability, be it regeneration, hard skin, king’s haki or whatever, he just grants it with no consequences.... it’s the embodiment of BULLSHIT... same as Blackbeard breaking rules of One Piece by eating 2 DF just cause it’s hype, with no logic
So yeah, it’s asspull for Kaido not Luffy... and the only reason he gets a pass is because he’s not the MC and because he’s got title of Yonko which is the most wanked title by fans