Am I the one assuming too much? Then just think about Buggy, he has no accomplishments bar being part of Roger crew. That alone basically was enough to give him the warlord proposal by the WG. Kaido was that with the difference he was also recognized as one of the strongest in the crew and even if we assume it was captured right after the God valley incident either way he already had the strongest zoan and the WG recognized that as well otherwise why send the greatest scientist Vegapunk to work on him and his df? So either he already was a big threat already or for sure it was evident he was a yonko seed. Yet he escaped without injuries.
No, he wasn't recognized as one of the strongest. He was brought up in the conversation because he is currently a Yonko, so it was obvious that Sengoku would mention him since a current Yonko was in the same crew as another two Yonko at that time, but he was merely an apprentice, he didn't say anything about his strength being great at that point.
If you give the quake quake fruit to a kid, he won't reach the lvl of destruction of WB. Or magu magu, or any other overpowered fruit. One needs many years of training and fightning with it. So it doesn't matter if Kaido's fruit had the potential of the strongest Zoan, if he was captured, he wans't the strongest at that point despite his fruit.
You are assuming a lot of things.
But the most safe assumption was that he was weak at that point when he was captured, not a fodder but still away from an Admiral.
Yonko didn't exist at that point (before 20 years ago). At that point it was Roger, Shiki and WB era. And 20 years ago Kaido merely had an army of 1000 men, while currently he has 30k.
It was early to say his potential (his own, not his fruit, for now his better feats in combat are without his DF).