I am convinced he is wrong, however, to put all the braking down to "intake drag".
I said no such thing. I never put all the braking down to intake drag. Just most of it. Anything mechanical experiences internal drag. Your car engine expends a great deal of it's energy not to power your car, but to overcome it's own internal resistance. Same for a jet engine. The net thrust that remains as useful energy for moving the aircraft is what's left over after overcoming the inefficiencies of the powerplant. Take away that thrust by deflecting it elsewhere, and while you may get some benifit from that thrust, the primary benifit is what's left when that thrust is removed. What's left is drag, and that's the chief component of reverse thrust.