In theory I agree. But learning something just well enough to get your foot in the door as a junior will have immediate rewards and then you can learn more deeply.
Most developers will never need to solve “hard problems”. They will be working on yet another software as a service CRUD app or a bespoke line of business app that will never see the light of day outside of their organization.
I’ve found studying architecture, soft skills, and my business courses (before I dropped out of graduate school) to be much more helpful in my career.
Here was a typical (not verbatim) exchange when I was a Front-End Architect interviewing candidates for my team back in 2012 or so:
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Q: Tell me about your experience with JavaScript.
A: I know JavaScript because I've been working in jQuery for a year.
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Further questions would isolate the fact that they've _only_ used jQuery. It was depressing, really.
I mentor developers to really strive to learn and understand the fundamentals precisely to stop propagating the idea that libraries and frameworks > fundamentals. Problem is, I've already started to witness history repeating itself when interviewing candidates and getting the equivalent answer above--but with "React" in its place.