It seems to me that the article is missing the point somewhat. There's absolutely nothing wrong with the em-dash, but most people never use it (I don't think I've ever used it), because it doesn't appear on most standard keyboards.
If you encounter an em-dash in an online discussion, most likely someone went to extra effort to include it, or it was automatically inserted, possibly by an AI.
There are other signs that you're looking at AI-generated texts, like lists of three, a certain turn of phrase, or vague generalities, but those are easier for a human to type than an em-dash.
Arguably there are two types of emdash users: robots, and bookworms with English or journalism degrees who actually had to learn to use oddball punctuation in the typographically correct way.
Shift option dash.
I went to the "extra effort" of learning the (trivial) Mac keystroke to insert an emdash, probably about 10 years ago. Now I use it so often and so easily that I can't even recall which keys it is---it's pure muscle memory. (that three hyphens thing there is translated into an emdash by Pandoc, so it has become my go-to when I'm not typing on a Mac)
I expect most people who use emdashes in casual writing are people who have done a lot of reading and a substantial amount of writing too (professionally or otherwise), who are also tech-savvy enough to understand that it's possible to easily insert symbols that aren't printed on the keys of your keyboard. These are the people you're filtering out when you decide to use emdashes as your primary signal for deciding whether text was written by AI or not, and I think that's pretty stupid. In your haste to avoid content written by AIs you're filtering content written by people who care about writing, which leaves content written by people who don't, and content written by genAI systems told not to use emdashes, which you will naturally be far less suspicious of because you're fixated on the emdash thing.
I generally think it's terrible that genAI slop is displacing human writing, however useful I might LLMs for other tasks. In that, I probably agree with a lot of the people using emdashes as a negative signal. But the fact is that this widespread attitude toward emdashes can only accelerate that displacement, by tarring high quality human writing as suspicious while giving cleverly manipulated LLM output a pass.