The main advantage of the Sabre concept is not only using outboard air as substitute for onboard oxidizer, but also as reaction mass. This allows to accelerate to over Mach 5 while still flying in airbreathing mode, using a fraction of the amount of fuel that would be required to reach the same speed on pure rocket power, and no onboard oxidizer. Once that speed is reached, the air becomes too thin for the air-breathing mode to continue functioning, and the engines switch to rocket mode, burning onboard oxidizer instead of air; but because the ship is nearly out of the atmosphere at this point, it doesn't take much to finish accelerating to orbital speed. Think of it basically as replacing the whole first stage of a conventional rocket with some jet engines and a sip of fuel for them.
