I'm not sure what stops AI from using this punctuation mark. They don't seem to scruple from plagarism or copyright violation.
In the unlikely event there was uptake, websites would regex emdashes into amdashes to appear human, AI would train on that, and we’d be looking for a different secret signal.
Problem is, it's only useful if you're using those specific fonts. Now I've gotta teach my grandpa how to install a font and switch to using it on every single device and application.