Comment by zamadatix

And that's why it should be interpreted as a one way signal. I.e. lack of spaces is a great indication a human wrote it but spaces sround an em dash alone is not a particularly strong indication it was AI.

Even for Americans, there are several style guidelines/modern preferences (particularly around web content) which don't guarantee the lack of spaces around an em dash. Hence even American LLMs using spaces. Ecen my natural em dash usage always included spaces as an American.