My understanding is that people have died with as little as one-third as much fentanyl as Floyd had, but that such a low dose would not be enough to kill most people. Even the much higher dose he had would kill many people, maybe most people, but would not be enough to kill everyone. So we can’t be sure that it was this that killed him.
But he had other drugs in his system, and he had a bad heart, and he’d had Covid, and he had been fighting with the police. So I think it highly likely that that combination was sufficient to kill him. But even if it didn’t kill him, it provided ample “reasonable doubt”. For a “guilty” verdict, you have to be sure, beyond a reasonable doubt, that it was Chauvin’s actions which killed him, and there’s no way you could be so sure.
(For this post I’m leaving aside the other two points I’ve made, namely Chauvin should have gotten off Floyd sooner, and the black cop might have been on his back for too long.)