Interestingly, in my client both show up as separate dashes. I suspect it's simply a ligature in the font used in your frontend.
E: Hm, no that is not it, copy-pasting the text from the web frontend copies the em- an en-dashes, so it's actually replacing the characters during rendering.
Oh really? So how do you prove no AI has been used? In the case you describe, there was at least some evidence that AI was used in form of the author's posts, but I regularly see people accusing things as AI generated based on vibes (see e.g. the comic author from the OOP. Publishing Krita time lapses and source files seems not to be enough evidence for some people)