This is great advice if you are early career and anxious about your CV and job security. I don't believe side-project-as-portfolio is the ideal strategy for career building for most people (though it may be for some).

However...

> you're early career, I get the idea that you are anxious about your CV and your job security

From reading the article, I didn't get this impression about the author at all. I strongly got the impression that the author is mid-career, and that they feel their career has benefitted from side-projects (I still don't necessarily believe this will be the case for most people, but certainly can for some).

What about the article gave you that impression?

Side note: I think outside of career progression, side-projects have many ancillary benefits and are very worthwhile.