A little NOS primer:
NOS is not flammable. It doesn't "burn" per se.
NOS (N2O) is a good carrier, by volume, of oxygen. When you spray NOS into the engine, the heat in the chamber during combustion dissociates the N2O, freeing up the oxygen. If you add more fuel, the freed oxygen oxidizes (burns) it, creating more power.
If you don't add enough fuel, you end up with a lean condition that causes very high combustion chamber temperatures and you also have free, unallocated oxygen that can oxidize things other than fuel: hot exhaust valves, spark plug electrodes etc. This is why you need to add extra fuel when the NOS turns on and why, if you don't, you can quickly wreck engine parts due to heat.
A second benefit is the very, very low boiling point of N2O. When fogged, it sucks a ton of heat from the intake tract making the charge density that much higher.
In my book, when you spray NOS, you are adding oxygen the engine wouldn't have ordinarily been able to obtain on its own. Not technically forced induction but not really normally aspirated either.