Yeah, we do know that much.
However, at this point in the story, it could be just about anything that caused that to happen. We have somebody can make a giant paw-shaped PAIN AGONY BALLOON with his SQUISHY MEATBALL PALMS, so.
Point taken. It makes more sense to me to think of Kuma's fruit as an air pocket (pad) fruit with which he can create giant air pressure bombs, absorb blows via cushioning, and shoot air pockets at people. Needless to say, it's a very bizarre power.
The thing is that several people ( I think you pointed it out, Gohn) have noticed him making a "Pika" sound effect, So a reflection or light based DF would be possible. Or, he could simply be using some admiral-level form of Soru or some type of rapid movement.
I really don't think the admirals were meant for Luffy (or any of the Strawhats, for that matter) to actually fight, though. I mean, Roger spent his entire career on the run from Garp. No one has said that Roger and Garp fought and Garp lost, only that Garp always failed to capture him, so there should be no reason for Luffy to do the same.
Also, remember when Garp was sliced by Morgan and didn't even wake up? This could be Kizaru doing the same: not reacting to a shot in the head.