I think the principle behind Carmack's letter is that factoring into multiple functions that all touch the same state is an illusion of reducing the burden on y

I think the principle behind Carmack's letter is that factoring into multiple functions that all touch the same state is an illusion of reducing the burden on your memory. They still interact with each other via that state. Pure functions, or your const T& situation, actually create smaller arenas of state. Splitting out multiple functions operating on the same object does not.