50.015% literally means that neither candidate is favored to win. Take out a coin, assign Harris as heads and trump as tails, now flip the coin a bunch of times - and that's exactly how often Harris or trump is likely to win the election
EDIT
Nate Silver just posted his final pre-election blog post and he explains very clearly that this is a dead even race. Either candidate is just as likely to win as the other candidate.
https://www.natesilver.net/p/a-random-number-generator-determined
He's not doing anything. His model is setup many months before the election, and then it stays completely unchanged until the election is over. He doesn't do any polling, he just runs his pre-set simulation model on the data that the pollsters release