I've found that people who say this sort of thing rarely change their beliefs, even after being given evidence that they are wrong. The fact is, as numerous people have pointed out, Word and other editors/word processors change '--' to an em-dash. And the "slang version" of an em-dash is "I went to work--but forgot to put on pants", not "I went to work - but forgot to put on pants".
BTW, "humans almost always tend to use" is very poor writing--pick one or the other between "almost always" and "tend to". It wouldn't be a bad thing if LLMs helped increase human literacy, so I don't know why people are so gung ho on identifying AI output based on utterly non-substantive markers like em-dashes. Having an LLM do homework is a bad thing, but that's not what we're talking about. And someone foolishly using the presence of em-dashes to detect LLM output will utterly fail against someone using an editor macro to replace em-dashes with the gawdawful ' - '.