So people who don't vote actually dont endorse a candidate. This is a bit of an advanced piece of politics, but the way voting in the US works is you vote for a candidate and if you dont vote for someone, then they dont get your vote.
Your logic rests on an assumption that the democrats are owed your vote, which they aren't. Had Harris won, then you wouldnt be saying the people who didnt vote passively endorsed her, but they would have done that just as much.
Those who don't vote are in fact doing the opposite of endorsing anyone, they are signalling apathy with the system.