It's not speculation. She's shown that she has can use it and its very strong since she can hurt and defend herself from Kaido's attacks. Yeh King has a hybrid form but doesn't have conquerors coating so idk why you bought that up.
Is it a good point. It's quite obviously a strength boost that's why Kaido has goes into it and then states he is now going all out. King is a 1st commander who should be comparable to start of Wano Luffy who got shit on by base Kaido.
Yamato can hold off Hybrid Kaido whilst King should get one shot by base Kaido
Zoro could cut Kaido without Conqueror's, so with Conqueror's he could at least do as much as Yamato.
And even Luffy did better than her.
So nothing shows that she had it for years.
King shouldn't be one-shot by base Kaido since his durable form can tank Zoro's Conqueror's infused attacks despite Zoro's regular attacks being able to cut Kaido.
Every 1st Yonko Commander doesn't have the same durability.
King is by far and away the most durable Commander in all of One Piece to the point that Conqueror's infused attacks can't even scratch him.
Even if Yamato is more powerful by far and away, it doesn't guarantee her the victory.
King could literally fly up and keep spamming his beam-like attacks like he did with Zoro.
Nothing is guaranteed.