I’d restate this advice as “be a specialist in several areas, start with one. Don’t be a generalist, while that is sometimes valuable, it’s not always.”
It is good advice. I’m doing pretty well in my career at 43 as an individual contributor. Next year I’ll be earning $500k easily (and I don’t live in NY or SF). I started as a coop student out of university which helped me focus on individual specialties. One of my very first jobs was as a database design analyst. I wanted to be a developer, but, i hit it off with this hiring manager in the interview.
When I started I had no idea about entity relation design, normalization, the great relational vs networked vs hierarchical database wars, data warehousing vs operational datastores, star schemas, reporting vs OLAP cubes, ETL tools, etc. I got very good at understanding a few of these tools [edit: specifically datamirror, informatica, Cognos, and Oracle RDBMS] very deeply, and became something of a technical expert on the topic of data warehousing, eventually meeting the pioneers of the field (Inman, Kimball) and eventually working for one of them (Terdeman). This was over the course of 5 years, with a sidestep into coding a large scientific test and measurement system on a Gemstone object database platform (a new area that benefited from my prior specialty). It was all a mix of serendipity and ability to focus.
My career moved away from data/analytics after that into middleware and REST architecture, and now cloud computing, and Kubernetes.
Tl;dr:
Knowing a few tools or subject areas super deeply comes across as having super powers. It really is about focus and curiosity.