I generally agree that betting markets should be taken with a grain of salt. They're only a spot measure of sentiment about who's going to win. And that's all they are. Trump supporters are touting them, naturally, because they're trending (slightly) towards a Trump win. But...
Hillary Clinton's futures were trading above $0.80 on Election Day 2016. And look how that turned out. All that it meant was that most people who were participating in those markets thought she would win -- which isn't surprising. Most people everywhere expected her to win.
But, FWIW, Polymarket isn't the only betting market showing the race at 60% expectation of a Trump victory. Pretty much all of them are. But that's likely to change between now and Election Day -- and it could very easily change in Harris' favor.