I don't buy the story about the traitor, it's just too pat. If you are going to spend a few days in a region with an army of tens of thousands, your own guys are liable to do some reconnaissance. No path that can hold thousands of soldiers is going to be that secret.
However, if, for some reason, the Persians really can't find a way asround Thermopylae, the blocking force could well hold them off a few more days. No more than that, certainly - there are limits to what human flesh will bear, and the intensity of fighting in that defile must have been insane. And even then, there is at least the chance that, with enough time to be frustrated, the Persians would try a different approach, maybe Inchon them. A heroic story no doubt, but it doesn't work as anything other than a delaying action.