I always use an em dash when possible when I should, and double en dash when I can't, just because I'm that kind of nerd. But it is the case that a double en dash on iOS autocorrects to an em dash, so I'm suspicious of the claim that em dashes are a tell for LLM writing.
Not in all fonts. In most monospace fonts, two hyphens will show with a small gap between them, for example.
I also personally prefer en dashes, surrounded by whitespace on both sides, over em dashes. Apparently some WYSIWYG software interprets two hyphens as an em dash, while other will interpret that as an en dash, so I'd rather just use the real thing if possible to avoid the ambiguity.
