I personally like the idea of Driago stuck in the warp, battling forever and getting summoned. To those who don't like it, I have a few points to clear up:
1. He can't survive unprotected in the Immaterium, even ships get destroyed.
That's true, but he is also the Emperor's Supreme Demon Slayer, the top of the entire order designed to fight Chaos for the Emperor. Perhaps, having been pulled into the warp in a weird way he became part 'demon', with his patron god being the Emperor, whether he knows it or not.
2. He shouldn't be accidentally summoned, as a summoning creates a portal, and demons stand on the other side waiting to get through.
True, and he's all that comes through. Given what we know about him, perhaps he shows up to kill some demon ass, kills all the demons waiting, then goes through himself.
3. He kills hundreds of demons and even carved his name into the heart of a demon primarch.
He IS the best Grey Knight, and if I am right he is also infused with a 'demon' like level of power from the Emperor, the very thing Demon's detest the most. Given that, I don't see it as entirely implausible that he would be able to best these odds. Holding off armies of demons, that's silly. Taking a demon primarch to his knees, when he is basically the equivalent, well that's not so far fetched.
Basically it's a little hard to swallow, unless we see him as the Emperor's first 'demon', using his great power to keep the body and soul of one of his most powerful follower alive. Add to this the fact that the Golden Throne is showing 'flaws' and it all screams the Emperor is about to burst forth into the Warp in a big bad way, and perhaps the dead Grey Knights and other Astartes will be summonable the same way demons are, while they wait for the final battle in the immaterium.
Just some fanboy stuff, but I'm enjoying it so far. Would I prefer they didn't basically make him a god? Yeah, but it's not terrible.