Are you talking about starting a fight with Sargos in New Garand? Yeah, that's a tough fight. Not impossible, but very unlikely when you first meet him. The game design is for you to agree to meet him elsewhere. The combat option just teaches some humility.
Beating Sargos in the arena is a routine part of the game.
Side-stepping the alien-centric nature of your comment, did you ever stop to wonder if Sargos himself is alien, or part alien? There are lots of aliens running around. Some are known wizards and sages, some are known criminals. It's well within the game lore to speculate that Sargos and other seemingly superhuman figures are in fact superhuman.
While not intended to part of my response, I guess we have to concede that if the world has one group of aliens, than it could have another group of aliens. And there's enough vagueness around the ancient sleepers (in the ancient temple) that one could speculate a wide range of things.
There's nothing in-game that says there's another group of space invaders, but there's nothing in-game that says there isn't either.
David The Developer has long supported the idea of keeping some things in the game a little more open to player interpretation. It's good world building, allows for a wider range of ideas, and also makes it easier to dove tail things in later.