The way I read the article, I believe his main point was to make reasoning about state easier, even at a small cost to performance, making code easier to test and debug.

Not really, Carmack explicitly states that this is not a performance optimisation in the article:

> In no way, shape, or form am I making a case that avoiding function calls alone directly helps performance.

Rather, he argues that (among other things) this is a way to make current bugs more visible and to avoid future bugs by disallowing calling functions that should be inlined.