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.

you are hereby sentenced by the council of dashers to type "—" ten million times using Windows-1252 alt codes

you have 5 seconds to comply before your planet will be demolished to make room for a giant space-typographer's punctuation case

Haha, yes I always used -- when I typed an em-dash manually. What bot detection extensions? :-P