So the poor designed system stays on the same sound level, even under load while the great designed system gets noticeably louder the more load it's under? I'm confused by your logic.
It gets a lot less confusing if you try to not compare it to a system with a naturally higher TDP and think about it logically. Yes a system should be much quieter and smaller when it has a much lower TDP and an external power supply. It should also be dynamic so quieter when not under load yes. A well designed system doesn't mean quieter than a system with a lower TDP. It just means well designed for what it is.