I can't speak for the person you're replying too, but I use -- for emdash for two reasons: I never remember how to type an actual emdash in linux/X11, and more importantly, I do most of my writing in Asciidoc which converts -- to an emdash automatically. It's nothing to do with bot detection or whatever.

But it does get me confused sometimes because in LaTeX (and other markup languages) -- gets converted to an endash whereas it takes three hyphens --- to make an emdash.