I'd never heard of red mercury before reading this article.
I don't think it's a widely known myth.
So, it is not really that obscure either.
edit: I just found a New Scientist article from 1995 which seems to take the idea somewhat seriously.
https://www.newscientist.com/article/mg14619750-300-cherry-r...
I suppose that claiming that something is a "myth" is a bullshit excuse journalists use for writing about something that is completely untrue, which they are not normally supposed to do.