I rarely see it self-applied, though. It always seemed to be a more HR-centric / economist verbiage, in the same line as "human capital" and "downsizing".

Whereas people actually call themselves "rockstar developers" "language ninjas" or "data scientists".

The term came out of management consulting (Peter Drucker IIRC) so it's not surprising it's more associated with manager happy-speak gobbeldygook than actual humans. Same with "content producer" (suggests undifferentiated glop, doesn't it?) or "thought leader".

Why? Isn't it just another term such as white collar worker?

It implies that people that don’t sit at a desk don’t have knowledge or use knowledge or advance knowledge and nothing could be further from the truth. In fact, my life experiences say it is the opposite.

It never implied that to me.

For white collar, soft skills are more important then domain knowledge for example and for blue collar work you'll need to be able to physically apply the knowledge.

That would leave the term to mean "your soft skills and handiness are secondary to your domain knowledge"