It took Blackbeard a year of him consolidating power, taking over territories and then beating the Whitebeard Remnants to be considered an Emperor.
Simply killing Whitebeard was not enough. Even with that, his unprecedented two DFs, him playing the World Gov like a fiddle to get a crew of monsters togethers, that was only enough to be considered someone with Yonko potential. He had to follow through on it and cement his position in the world.
That was good writing. Kid and Law chucking Big Mom off an island and suddenly being Yonko isn’t.
I think the WG is taking the bigger picture into scope here. They aren't looking at this like we are: they weren't there. To them Monkey D. Luffy (the grandson of the greatest Marine known in the series and son of the most notorious man in the series, who beat Kaido in 1v1 combat), Eustass Kid (a man who has stolen from Yonkou and challenged multiple Yonkou and survived, before knocking Big Mom into Wano's underbelly) and Trafalgar Law (a former Shichibukai with the DF the WG desired most, who helped defeat Doflamingo, shut down his underworld operations, and then fought against Kaido and aided Kid against Big Mom) ALL JUST EXECUTED A RAID THAT DEFEATED TWO YONKOU.
This isn't Black Beard killing a dying Whitebeard. This is Kaido in his prime and Big Mom as full health/near her prime getting defeated, straight up, by an alliance between three Supernovas. That is more terrifying than anything Black Beard has accomplished, as far as the WG is concerned, because now the balance of power is completely broken.
Three Supernova just led an alliance to successfully beat two Yonkou. What if Luffy, Law, and Kid join forces? The idea of Kaido going to Marineford was scary enough to the WG. Imagine three Yonkou, one of which is like a little brother to Shanks, joining forces and making Wano their pirate fortress? That is a nightmare scenario for the WG and Marines.
I understand that a good number of people hate how Kaido and Big Mom were defeated. But, from the WG's perspective, the defeat of two Yonkou is defenitely a giant feat in Luffy/Law/Kid's favors.