* How is drilling mental math any less mechanical
than the unconscious familiarization/ingraining
of algebraic concepts of this app?
There is a difference in that with mental arithmetic you have, or can have, a direct connection with an underlying reality. This seems to be entirely divorced from any reality, and the rules can, if you don't already know what's going on, appear completely arbitrary.
I can picture asking an adept "why do you put the same thing on each side?" And getting the answer - "Cos that's just how it works."
Yes, it is how it works, but it's not just how it works. There are reasons for the rules, and underlying models for the rules. It's not arbitrary, it has evolved over centuries to have purpose.
It's not "just" how it works. When you have the equals sign between two expressions you are saying that instantiation of the variables must result in quantities that are the same. When you modify one side then you must make the same modification on the other side in order to retain that property. there are reasons for things to be the way they are.
I can picture asking an adept "why do you put the same thing on each side?" And getting the answer - "Cos that's just how it works."