I don't think you're wrong in general, but in one specific case I actually can give an example where me not knowing how a hash-map worked was really bad. I was working on a very limited set of memory for a small embedded thing when I was an intern, and was using hashmaps all over the place because they're magic, and kept getting out of memory exceptions.

I learned later that hashmaps are inherently O(n) (or O(log n)), or they take a lot of memory; internal arrays can get super huge if you're not careful.

In this particular case, an ugly nested for-loop was the immediate solution, and eventually I was able to cheat a little and have hard-coded integer indexes and was able to use an array.

Anyway, your point is valid, I just figured I'd give an example where knowing the internals for a hash table would have saved me a lot of time.