Well the implied idea is that the "future humans" not aliens, are manipulate time space using gravity. So they use the black hole as a portal and a battery. And the fact that they put him back after 75 years (I believe) have passed in "normal time" suggest he spent like a few minutes in the black hole. I'm not gonna do the entire math, mostly because I don't remember the mass of the black hole. So basic they manipulate the gravity in the black hole to create a space were he could survive and interact with the universe at large in normal time.

This works because the time dilatation inside the black hole stretches back in history, so "deeper" you go further back in history you ca see, and send messages, at least in the movie, we don't have any idea it would be possible. So after he sent the message they created a wormhole to bring him out, but the trick is that they can only bring him back and in the normal time, unaffected by the black hole, so basic he travel in the future from his perspective. So in this regard they respect the general relativity.

Is pretty hard to imagine and explain how this works, is hard core physics and I'm not that good at it :))