Total BS. Irish War of Independence is a good example of guerrilla warfare beating the oppressor, and the British did not subscribe to 'laws of armed conflict' in the slightest:
https://en.wikipedia.org/wiki/Bloody_Sunday_(1920)
https://en.wikipedia.org/wiki/Bloody_Sunday_(1920)
People are generally not willing to die when they know defeat is certain if life is an option. If the British had been willing to be as brutal as Cromwell to retain Ireland they’d have won. They weren’t.