Donnie's goggles aren't quite that capable of identifying such properties, or at least not in ways he'd be able to figure as pertaining to some weakness or another. It's mostly by magical auras, but he can see that strange darkness surrounding and attaching the larger figure to Akechi.

"O-oh, no, I wasn't expecting a demonstration, don't worry about it," he says as he pushes his goggles back up.