A "parenthetical dash" (the semantic meaning) can be typeset either as an em dash (a typographic meaning) without spaces, or as an en dash (a typographic meaning) surrounded by spaces. And the latter is often referred to as an em dash (as a semantic meaning), since basically everyone uses that to mean "parenthetical dash" which is the correct term but a term that virtually nobody uses.

Its correctly used, just not common in American English. <https://www.chicagomanualofstyle.org/book/ed18/part2/ch06/ps...>

Link is behind a login wall.

Wild, apparently my institution pays for that, sorry for the dead link.

<https://cmosshoptalk.com/2020/06/09/en-dashes-the-editors-ma...> quotes the relevant section here.

I can't find another source for the CMOS.

Thank you for the link and the correction.

The Internet Archive has at least old versions of CMOS.