He's not pulling it out of his ass.
It's from here. Also, what hornet said isn't really attacking Mantle, so back off with the aggressiveness will ya? All he said is that the way Oxide decided to show the difference with Mantle is to disable or enable motion blur. I quite agree with him, since I'd rather see the demo bump the unit count to 10000 and show the difference between Mantle and Oxide.
I do disagree with the rest of his post though. I've checked Total war, I've checked supreme commander, I've checked all the other high unit count games. NONE of them could do it with the performance the Nitrous engine could. NONE of them could have 5000 units all at once, on a single screen, with all the weapons going off and still run at 30 fps.
Though, as we all know, the Star Swarm demo is coming pretty soon. I'd much rather wait for that and test it for myself.