I expect that what they found was something analogous to the Halting Problem. I don’t think the rules of M:tG are quite Turing-complete, but they may well be complex enough to make something like that an issue, anyway.
Of course, that’ll only come up in extremely contrived corner cases that are unlikely to come up in any real game.
Oh, it should also be mentioned that the computers’ inability to conclusively solve M:tG does not imply that humans will always be better at it, because humans can’t conclusively solve it, either. It would still be theoretically possible (though still a long ways away) to create a computer better at it than a human.
Note that by “better at it”, I mean at all aspects of the game: I wouldn’t be too surprised if you could take a simple deck like Relentless Rats or green zoo, and teach a computer to play it better than a human. But being able to construct a good deck, that’s another story.
Yeah, octopus, that’s the one Jophiel linked, and which I’ve since read through. It’s exactly the halting problem: One of the official rules of Magic is that if the game ends up in an infinite loop, it’s a draw… but the problem is, there’s no general way to tell if a computer program is in an infinite loop, or if it’ll end eventually, just not yet. And someone figured out a way to implement a computer, capable of running an arbitrary program, in a Magic game, set up in such a way that if the program eventually halts, then one player will win. Set up the game that way (which they found a way to do on the first turn, if you get just the right hand), and program it with something like the Goldbach conjecture whose truth we don’t yet know, and the game is over, but we don’t know if it’s a draw or a win.