When you capture a yonko or at the very least one of the main assets of Xebec it is logic you don't send fodder to kill him. And even if you do after they fail time and time again despite the seastone cuffs you don't need Einstein to understand you need something more. At that point or they have not the power to do it or were so fool to not even understand the situation. Anyway BM as well referred to him as a thing probably impliying his invulnerability as well (be it because she tried to kill him or witnessed it when they were both under Xebec) so it is not strange or a shame for an admiral to fail as well.
He wasn't a Yonko at that time, that's the point. And neither he was a main asset of Xebec, he was an apprentice in that crew.
He barely had an army of 1000 men 20 years ago (compare it with what he has now, with BM's army and with WB's army) and Yonko = 4 Emperors -> Shanks became one 6 years ago, so only 6 years ago we have for sure the Yonko characters.
He didn't have at that point in time the prestige and probably the strength he has now.
You are assuming too many things. The fact is that his capture was probably between the incident in God Valley and before he came to Wano 20 years ago. A time range of 18 years.
He could have caught 37 years ago for example, and at that point he was definitely a noob in strength.
So for now, there is not any proof regarding something like what you said.