That's not really true and the radiation sign is a good example of that. Even though it was a new symbol at the time, because it visually looked similar to the related phenomenon it was possible to find meaning in it without prior knowledge. Not necessarily the right one and that's a part of the point. If your icon looks visually similar to something else, people might incorrectly infer a different meaning.
It's hard to argue the radiation symbol inherently indicates danger though. That our seemingly natural reaction to it is "beware" can only be the result of learning the meaning of it. Unlike, say, signs showing "danger to cyclists" by literally depicting a cyclist flying off their bike (https://www.roadtrafficsigns.com/signs/cyclists-falling-on-s... - actually the only place I recall seeing signs like this was NZ).
