That's the exact point of HM.
Abstractions come from the knowledge gained after wrestling with problems, not as a form of premature optimization.
And this is especially true in games because you need to get all abstractions right otherwise the game will be limited and/or not performant enough.
In many other programming fields you don't get punished for adding a few abstractions that are not strictly necessary, so usually other programmers are used to abstracting things away fairly early (eg in OOP: properties vs public variables, ...) because they're free as long as you don't overuse them.
Also you should keep in mind that the stream is educational so Casey sometimes takes the scenic route before getting to the point, and he's both implementing the engine and designing it, so sometimes he doesn't know yet what he wants exactly.
And this is especially true in games because you need to get all abstractions right otherwise the game will be limited and/or not performant enough.
In many other programming fields you don't get punished for adding a few abstractions that are not strictly necessary, so usually other programmers are used to abstracting things away fairly early (eg in OOP: properties vs public variables, ...) because they're free as long as you don't overuse them.
Also you should keep in mind that the stream is educational so Casey sometimes takes the scenic route before getting to the point, and he's both implementing the engine and designing it, so sometimes he doesn't know yet what he wants exactly.