[conor-]

Foundational skills can mean different things. In your example of React/Redux the foundational skills means learning things like how the JS Eventloop works, browser APIs, Layer 7 networking protocols, etc. i.e. the underpinnings of what React/Redux relies on to work correctly rather than just learning how to write a bunch of stuff on top of React without fully grokking the what/why of what's happening.

All of the aforementioned skills are broad and will allow you to pick up new things a lot faster because you understand the foundations vs just being shoehorned into being "$Framework Engineer"