The ZPinchFusionEngineAirospike engine, shuts down due to maxAtmosphereDensity being too high. Seemingly it uses the same code under the hood as (for example) the Vista Engine.

if (vessel.atmDensity > maxAtmosphereDensity) ShutDown("Inertial Fusion cannot operate in atmosphere!");

The problem with this is both the error message and/or the behaviour. I can understand (although it is odd, it being an airospike engine) it not working when the atmosphere is too thick (although it should either throttle or overheat gradualy instead of just shutting down). But the error should not be the same as the Vista engine.

it seems someone stitched the Vista code onto the ZPinchFusionEngineAirospike.