I'm not sure I'd call this luck. If Trump had displayed confident and competent leadership in the face of COVID and managed the crisis well, it would be a huge mark in his favor going into the election. Instead, he (we now know) knowingly misled the US, and basically put his head in the sand when it came to guidelines, PPE, and everything else.
So instead of being something that could bolster his case, it turned into one of the easiest arguments against his presidency.
Also, Biden's points at the debate were extremely straightforward. I'm not sure where he'd have made a fool of himself, because he seems sure of his platform and who he's talking to (the American public). It's kind of hard to go wrong with obvious stuff like readdressing climate change, tightening our handling of the pandemic, etc., when your opponent is doing none of those things.
I don't think Biden is the best debater in terms of tit-for-tat, but Trump doesn't have any solid platform of his own to put up against him, so it doesn't really matter whether he interrupts him or not.