But Big MT is in the west. In order for the Legion to get there in any force, they would have to defeat the NCR at Hoover Dam, (thus ending the game.) After the Hoover Dam victory it would take the Legion at least a few years to recover, gain complete control over the Mojave, and get a large enough force together to invade Big Mt.
They wouldn't have to invade in force. I'm only suggesting that a small Legion party could make it.
The point is, Elijah, Ulysses, and Christine all made it in, and out. So, it is conceivable that someone from the Legion could, too.