Well, it is the Pro documentation, I wouldn't expect most of it to apply anywhere else. With Arcade, there's Pro, and there's everywhere else. When you're in a web context, it really matters what profile of Arcade you're using. (https://developers.arcgis.com/arcade/profiles/)
The labeling profile is strictly looking for a text string. It's unfortunate that Pro handles that string differently from other contexts, because that can be pretty misleading. But everywhere else, the profiles are consistent with how they handle things like HTML tags, etc.
- Josh Carlson
Kendall County GIS