Only because people don't understand probability and statistics. 538 gave Trump a ~30% chance of winning. The fact the people seem to think anything less than 50 equals 0 is a problem with people's understanding of statistics, not the statistic itself.

The assertion you're responding to was not about Trump's odds in an absolute sense, but relative to other pollsters and forecasters. So pointing out the precise value FiveThirtyEight assigned to that outcome doesn't refute anything until you compare it to somebody else's number.

The GP is saying 538 did better compared to other news outlets. You need to also provide a source for another news outlet giving Hillary less than 71.4% if you want to show that the GP is wrong.