I assume that the argument is that pros can compensate recoil to some extent (i.e. the game doesn't just add increasingly random offsets to the trajectors but actually moves the crosshair and you can move it back if you're fast enough). If so then a cheat only has to simulate sufficiently realistic inputs based on the pixel outputs of the game, which is practically undetectable if you do it on external hardware. The only advantage the game devs would have is a larger amount of data they can use to determine what passes as realistic. And how are they going to identify false positives?