I will note that by saying non-monogamous people "can't commit infidelity" you're using a very narrow definition of infidelity.

Polyfidelity (where you have more than one partner but are not allowed to sleep with anyone outside those relationships) is absolutely a thing, and you acknowledge that.

Thing is, there's this myth that polyamorous people can't cheat, but it's really not true. It's just that the definition of cheating might be negotiated within each relationship rather than being the traditional rule of "If they aren't your primary partner it's cheating no matter what."

That said, you've attempted to understand a complex issue very well.