If you are an alternate for Pole you have a slightly greater chance of being called than at McM. People do get here and decide (or find out) they can’t handle the altitude and go home. Not common, but it does happen. There is the story of the person who went through the entire process to get to Pole, stepped off the LC-130, looked around and got right back on to be taken home.
But being an alternate is pretty good because you have a leg up during the next hiring season. Keep in contact with your hiring manager.