On reflection I agree.

"Metaphysically impossible" is a rather strong requirement if taken strictly, and I think strictly is the right way to take it when talking about metaphysical possibility.

So if you think there's a possible world where human actions just happen by chance to line up with some mysterious experiences and zombie actions don't, then it's metaphysically possible.

I'm not sure whether Dennett's answer should really be read as "zombies are metaphysically possible" but I'm now convinced this whole question is silly.