I remember participating in a small thread on how to type an em-dash, on different OS's. It was in March 2023, so before the em-dash meme had started—it was an innocent question then.
I think they meant after the release of ChatGPT. If someone never used them before and now uses them all the time it might indicate that they're using ChatGPT... or it might just mean that they learned how to use them after widespread discussions about it.
More generally any measurable feature of writing that underwent a significant change in frequency around that time would be interesting to look at. Looking at frequencies across the entire post dataset would suggest likely candidates, which individual people could then be tested against. There would be lots of confounding factors and red herrings though -- like the word "ChatGPT" itself!
Even more interesting is the likely increase in emdash usage by those not using an LLM, but merely imitating the writing they see subconsciously. There was a evidence that chatgpt is shifting the frequency of use of some uncommon words and phrases amongst non-users.
I missed the point of the leaderboards completely. It is to show exactly that when you get blamed for using AI to write. You can point out that you already used it in 2009 or whatever. For that it is very useful yes :-)
