I've tried to poke a little bit around but do not fully understand why this works. Beforehand, did MC compile all transformation rules, regardless of a world needing them or not? And how does setting the executor to void solve this? Is this just the pre-game one that compiles them?

Beforehand, did MC compile all transformation rules, regardless of a world needing them or not?

Yes.

And how does setting the executor to void solve this?

Well, it doesn't set it to "void". It just doesn't execute the pre-caching tasks. Since these tasks are executed in a thread pool with a good chunk of system resources allocated to it, it tends to make game startup faster, especially on low-end hardware.