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...