I presume whatever Musk said was referring to something legitimate, but to put a finer point on your comment: in normal jet engines, only a fraction of air actually goes into the combustion chamber. The rest is just pushed out the back (faster than it came in) by turbine blades that are driven by that combustion. Indeed, modern jet engines are "high by-pass" which I think means that the large majority (>80%) of air isn't part of the combustion.