Very true, but I would advice against piling on dev skills and also invest a little time by adding a few skills relevant to software development as a whole.
What I mean by that is, instead of learning yet another PL/DB/editor/framework maybe learn a little about excel, planning, estimation, budgeting, stakeholder risk, software development lifecycle, automation, human factors, project management, requirement specification, leading a team, reporting to a product manager, customer support, marketing, sales, conflict resolution, etc.
Think of it in terms of movie production. You can be a specialist (let's say an editor or 2nd camera assistant) and only focus on doing that well. And that's fine.
But from a team perspective you will be even more valuable if you know a little about the whole process and challenges of other specialist on the set.
The reason being that: 1. It enables you to communicate in a more efficient way because you have some understanding of the challenges and needs of your colleagues – "how can I make the job of everyone else a little easier, so that we all win in the sense of making a better product?"
It enables you to express your needs in the language, terms, and possible consequences of your colleagues.
From a career perspective, learning a little about the areas outside PLs,DBs, networking and devices, could make career transitions (for example to management) much easier later in life.
Or to put it in one sentence: Study the game, to improve at your position.