The reason the enum is so nice is that it works as a terrific language-supplied abstraction that covers up those bit manipulations. It's very nice to get those abstractions for free like you do in Rust, but it can't be optimal for every specialized use case.

This new code also supplies similar abstractions. That actual specific code is much harder to reason about, but most users--and even the next person who works on the interpreter--simply won't care, or even know what is going on underneath the hood. The abstractions provided by the author do that work and apparently do it cleanly.

For most use-cases, that extra hand-written code isn't worth it. But in specific cases it can be, and the author has actually measured the value and determined that it is.