Never said Diego would take GSP down. By all means, GSP has great wrestling ability and he looked good tossing Serra around (then again, tossing somebody who is almost half your size will always look good). But what can he do AFTER he takes Diego down? He can't roll with Diego and if Diego wants to get back up, I'm sure with his scrambling ability, he will.
Diego has always had good, crisp striking. Thing was, he looked so out of place doing it. But against Luigi, he looked very comfortable and threw in some kicks and knees. GSP doesn't have the power to knock him out, can't pound him out on the ground, and more than likely, won't catch Diego in a submission.