The incentives to use bots are many.
This use was one of the first things that occurred to me when LLMs started getting genuinely good at summarizing texts and conversations. And I assume a fair bit of this has always happened on HN too. I've never moderated here obviously so I have no first hand insight but the social conventions here are uniquely ripe for it and it has a disproportionate influence on society through the dominance of the tech industry, making it a good target.
Lots of dumb blogs from unknowns about vibecoding entire products in a weekend using specific AI slop generators from new startups that are getting to the front page lately.
remember: 1) those accounts that are causing 10x as many em-dashes are the dumb AI accounts. The smart ones are at the least filtering obvious tells from the output. They might even outnumber the dumb ones.
2) Also, a lot of people are real, but using AI to make themselves sound smarter. It's not necessarily completely nefarious.
Using em-dashes as an estimate has to result in a bunch of undercounting and overcounting.