That section of the article unfortunately conflates two types of not switching off.
* Late at night, quick check of my email or Slack and maybe even reply (either to get it out of the way or to show off how hard I'm working).
* While I'm in the shower, thinking about some interesting logical problem (can be something mundane like how to clearly express something in a report).
What is healthy and what isn't is subjective. But most would agree the first is not healthy, while some would say that the second is OK. Personally, I think it's fine (nice even) and certainly doesn't interfer with social or family life.
You (comment I'm replying to) have added a third which is quite different: "chronic workers" are those that are flat out working for extended hours. That's not really the same as not being able to switch off.