That Spectator article (a) is grasping at straws where Trump's chances are concerned, and (b) doesn't seem to understand the concept of probability.
I didn't see the 538 prediction for Brexit, but I'm sure it was given as a probability rather than just a yes/no prediction. Let's say he said there was a 70% chance of Brexit being rejected. Technically that's a prediction that Brexit would be rejected, but it's allowing for a 30% chance that it would pass. That's far better than just guessing one way or the other and getting lucky (or not).
Everyone gets pissed off when the weather forecasters say there's only a 10% chance of rain and then it rains. That doesn't mean they were wrong - it SHOULD rain 1 out of 10 times when they say that.
Predicting City would beat Southampton the other day doesn't mean you were 100% sure it was going to happen (i.e. no other result was possible), it meant that if they replayed the game 10 times, you think we'd win most of the time, which is probably true, even though we didn't win this time. A City win was still the best prediction based on all the available data, and any other prediction got a bit lucky (which is why any other prediction won you more money than your bet at the bookies).
Similarly, all the available data suggests that Clinton is very likely to win the election, so in that sense you'd have to predict that she'll win, even though Trump is still given a 15% chance by 538. That 15% acknowledges that polling data could be wrong (but probably not by that much), that something new and disastrous could happen to Hillary (but it's not likely at this late stage), or that something else no one's even thought about could swing the election in favor of Trump (again, not that likely, hence only 15% chance for Trump).
It's no mystery why a Conservative magazine like The Spectator, or Trump himself and his surrogates would deny the reality of the polling data - perception matters, and if his supporters think it's over, they're less likely to bother voting. It's all reminiscent of that spokesman for Sadam Hussein during the Iraqi War who was predicting glorious victory over the Americans right up until the moment they took over Baghdad and pulled down that statue.
