That was always my understanding and pretty true overall, but there maybe a situation that doesn't fit that, for example having hot air coming into the cylinder and the a great glob of fuel stuffed in.
I think the problem is if you were to leave the carb heat set to hot with full power applied, rather than a momentary issue just after power application.
If you have all the heat being produced by having the throttle wide open AND carb heat hot, then I can well imagine problems, especially as the more power you have applied, the hotter the exhausts will get, so the hotter the carb air will become....... I can definately see this potentially leading to pre-ignition, which then goes to detonation. And this being at relatively low speed, so the cooling airflow isn't massive.
I think they are covering their backsides about prolonged application of full power and carb heat hot, the next question is how long would it need to be to before this could become apparant?
I can just imagine the engineers sitting around a table trying to answer that and then someone saying, "forget it, let's just write the POH to avoid this situation in the first place."