Li Wuji would've only trusted Li Yao's "credibility" if his brain had been damaged because of sinking in the septic tank for too long.
However, having lost his entire fleet, he did not have a second option.
Also, according to his secret observation in the Blue Sky Market, what Boss Bai was trying to build was indeed not a bloated, gigantic regular fleet, but a rapid response fleet that was small but elite. As a result, abundant starships would certainly be outdated. They could be used to establish a mediocre fleet that was commanded by someone.
In that case, why couldn't the "someone" be him, Li Wuji?
Revenge was a dish best served cold. Whether or not he was to settle the scores with Boss Bai for his insult, there was no need to end on bad terms with the guy right now. Instead, he needed to cooperate with the guy without reserve and recover his fleet first.