Symbols are far, far less translatable than words. We were unable to translate hieroglyphs with any real success until the Rosetta Stone was discovered, and the pictograms used could be matched back with known writing systems.
There are symbols today that you and I would instantly recognize but will be meaningless to a child born tomorrow (the floppy disk icon, for example). Symbols and pictograms can be repurposed and reused: indeed, the article gives the example of the skull and crossbones, which has significantly changed meaning over four hundred years.
The cargo cults you're talking about have come about in years and decades, not millennia. The timescales being talked about here are far, far different. If anything, you're not giving humanity enough credit for its ability to evolve over the next 10,000 years.
Once some individual humans get sick or die. Even very primitive human cultures in the vicinity of nuclear sites would very quickly notice a pattern. Any symbol that was used consistently near nuclear sites would be come to be associated with danger.
In a very primitive cultures with no knowledge of science, visiting the sites with these symbols would end up becoming some sort of religious taboo.
The argument isn't the the meanings of symbols can't change. But rather that humans are smart enough to notice patterns and communicate them with others.
Even assuming there is some sort of civilization ending event, and specific knowledge of nuclear technology disappears.
People are certainly smart enough to recognize a pattern of distinctive symbolism and learn to associate it with danger over time.
A good example is cargo cults that sprung up in the Pacific Islands in WW2. Groups of people without much previous contact with modern civilization learnt to associated 'airplanes' dropping aid supplies with food. Despite having no previous knowledge of what planes are and how they function.
I imagine future primitive cultures would learn to associate the nuclear hazard symbol with illness in a similar manner. And would probably end up constructing their own mythical narratives around the symbols.