Related article posted here https://news.ycombinator.com/item?id=46133941 explains it: "Within the A.I.’s training data, the em dash is more likely to appear in texts that have been marked as well-formed, high-quality prose. A.I. works by statistics. If this punctuation mark appears with increased frequency in high-quality writing, then one way to produce your own high-quality writing is to absolutely drench it with the punctuation mark in question. So now, no matter where it’s coming from or why, millions of people recognize the em dash as a sign of zero-effort, low-quality algorithmic slop."