National polling was accurate, but some of the state polling wasn't even close. Wisconsin and Michigan were off by a sizeable amount. There's no doubt he's down in the polls, but I don't think it's impossible for Trump to trip towards the finish line like he did in 2016.
Due to huge numbers of early voting it's getting increasingly unlikely with every day. For example if FBI announced an investigation into whatever Fox News is accusing Biden of, it would be much less likely to have the same kind of impact like Hillary's e-mails did because so many have already voted.