Can you give some anecdotes of where "understanding how any important/relevant technology works gives you an unbeatable edge over most of your peers"? Besides being able to pass a job interview. Naively I would assume that being able to apply these techniques (understanding cross validation, overfitting, domain expertise, etc) would be far more important than knowing the underlying logic of how to write them from scratch.
I think that it's way easier to apply something when you understand how that something actually works.
i.e. There is a difference between being able to re-implement an algorithm, and understanding why that algorithm has the behavior it has. You'll probably write cleaner and more maintainable code, quicker, in the latter, and be able to make modifications as necessary.
That's an easy claim, but putting more time into learning foundations means you have less time for applications. Moth pure mathematicians are poor web developers.
For work a month ago I implemented stochastic gradient descent in some arbitrary directed graph of operations (I applied a skill I learned from neural networks in a system that had nothing to do with neural networks) which required me to know how to take derivatives of a loss function with respect to parameters. I did all of the math on paper, and then coded it and it worked (to be frank this surprised me). It often may be hard to see the immediate gains from understanding things in a more technical, low level fashion - but I think in the long run they give you a great deal of power.
