My understanding is that that's because they were trying to do a structurally homologous port from Zig to Rust, precisely to keep their mental model and not change "too much" at once, and then they plan to refactor to make it safe Rust later.
it's clear that as of the time of this merge, no human has read any appreciable fraction of current mainline bun, so it's not particularly clear how much of a "mental model" exists anymore.
