Okay, I saw in the comments (both here and on the TED site) that Deutsch's point was that good explanations are "hard to vary", but I didn't understand what that means.

So I finally saw the talk (after skipping most of it to get to the explanation of explanation), and it turns out Deutsch just means "lacking unnecessary details" when he says "hard to vary". Which is just the standard point about the conjunction fallacy and how each detail makes your explanation less plausible.

Nothing new here, sorry :-/