Earlier quoted context omitted.
It is an en dash, not an em dash, since it's about as wide as an n. Some software substitutes a double hyphen -- with an en dash rather than em, and use the triple hyphen --- for the em dash. Perhaps Hacker News' title formatter is one of them.
Okay, edit submitted, but that is extra weird , because the double hyphen is a convention or placeholder for an emdash . When a transformation takes place, it becomes an emdash. There is no reason to transform it to an endash. I don't know any software that would do that. Checked with an LLM, too. That makes no sense at all!
TeX uses -- (double hyphen) for producing an en dash and --- (triple hyphen) for an em dash, and that's pretty darn longstanding and well-established. And FWIW, the English writing conventions that I learned use em dashes as punctuation without an adjoining space—like this—but allow en dashes surrounded by spaces as an alternative – like this – so I frequently see spaced double hyphen used as the ASCII equivalent of dash punctuation and interpret it as the latter. I've never personally heard of double hyphen for em specifically, only either as en specifically or as a sort of ambiguous whatever-dash.