Dependence on the charity of the river is mostly a reliance on luck. The charity of the river is simply a reason why paddlers usually end up finishing a run instead of stuck to the bottom of one. Even the most treacherous rapids and drops have a tendency to wash things into pools or to the bank. When they don't, and fortunately this does not happen more often than it does, they kill or injure. If the physical laws of rivers changed and all the molecules, for instance, sucked instead of blew, requiring skill to be tantamount and the charity of a river unfactorable, there would be few people paddling whitewater.
Charitable outcomes raise false confidence, and that is dangerous.
Good advice because it implies the obvious, that the Class II water is the less dangerous. But for spunky paddlers with an attitude, paddling Class II or III water wrongly shows to them inferiority. Going up, up, and away into harder rivers before you are prepared is the quick way to the bottom of a hole. Getting in touch with the paddler within is about thinking for yourself. It's about telling yourself that it's okay to be a beginner, that all good things truly do come in time.