It annoys me that recently people seem to propagate the wrong use of em dashes. There is no space—an em dash is practically never typeset with spacing. You use an en dash in European typesetting – they have spaces, which your typesetting system hopefully adjusts to sane values. They're not like the spaces between words.
At least, that's how I learned it. Everyone seems to disagree and write em dashes with spaces nowadays.
Agreed that it (used to be) less common. AP style (like the article mentioned) uses spaced em dashes, and I think that style of spaced em dashes are more common in writing today because that's what Claude (particularly Sonnet) likes to use (based on some half-baked investigation I did a bit ago [^1]). OpenAI models generally uses the American-style unspaced em dashes, which is why that particular style looks LLM-y to people now.
edit: Another comment recommended https://www.pbjstories.com/blog/spaces-around-em-dash which does an awesome job of diving into dash usage by style guide
[^1]: https://will-keleher.com/posts/your-dashes-suggest-which-mod...