Yeah, but it's not exactly clear that ONLY mutants would be at that spot on the moon. Perhaps Storm invited the X-teams, including Alpha Flight as allies, to assist in the battle. Alpha Flight sent 2 members, regardless of their mutanity.

When Magneto said, "Humanity has always longed to banish us mutants!", maybe "us" meant "me and you" meaning, just him and Storm.

Can't explain Juggernaut, maybe the same thing, Magneto just invited a bunch of baddies and their friends and he showed up.