80% is not a particularly high number. If a .200 hitter comes up in the 9th inning and needs a hit to win the game, it's probably not going to happen, but there is a decent chance it will. The people who were wrong were the many who said Trump had no chance to win based on that 80%.
I'm not making excuses for Silver. I think his time has passed. He was a very influential man in the field for a number of years, but isn't so relevant anymore.