@[email protected] This is not in reference to the radioactive potassium content of bananas. This is in reference to the internet trope of adding a "banana for scale" to uploaded pictures.
@[email protected] you're the one missing the joke, this has nothing to do with banana radioactivity. I'm aware that bananas are used as an equivalent dose measurement of radiation. This has nothing to do with that.
There's no 'actual science' behind the joke. Bananas were used as units of radiation in cases where radiation output was negligible, in a humorous effort to make readers realise that, well, radiation output was negligible. Instead a bunch of undereducated people took it to mean bananas were hazardous radioactive fruit.