>Can you point me to a single example of internet discourse from the last 30 years where a human used an em-dash unironically?

Thousands upon thousands.

>I'm looking for an example of human created discourse online. The crux of the allegation is that normal meatbag humans don't use an em-dash when conversing with one another online

Meatbag humans whose education failed them don't. Other humans did and still do, from Usenet to Substack, and from Slashdot to Hacker News.

Here's a random PG essay sprinkled with 23 em-dashes:

https://paulgraham.com/greatwork.html

Here's a post from idlewords, 13 of them:

https://idlewords.com/2025/02/the_shape_of_a_mars_mission.ht...

Here's the current top HN post, 13 of them:

https://aaronson.org/blog/square-theory

57 in this antipope post - many by Charlie, equally many in the comments:

https://www.antipope.org/charlie/blog-static/2024/12/storm-c...

And those are genuine em-dashes, the character.

Way more people, in posts, comments, etc. use en-dashes and hyphens as em-dashes (just because they don't know how to quickly insert proper ones, or aren't aware there's a typographic distinction, but do now the use of dashes for parenthetical statements and asides.