How would a single inboard brake work unless the axles were locked together?
You are correct, it wouldn't worked unless it was a locked. It works on a go-kart exactly that way, but with an open diff it obviously won't work. While you don't need as much brake on the rear axle as the front, if you only use one brake it's going to have to be a lot bigger than the brakes on each rear wheel.
Geez, take it for what it is, it's not going to lap like a Red Bull, Ferrari, McLaren or Audi, it is what it is.