Even if you “rely on countless unsafe blocks”, unsafe is additive, it gives access to additional APIs which are not checked. It does not disable affine types, the borrow checker, or send/sync traits. Unless the entire codebase is unsafe (e.g. fresh out of c2rust) it’s very hard to not have more guarantees.
And because unsafe is generally highly local or localizable reasoning (conventionally backed by safety justifications) it really is quite reasonable to go plugging at it, or task an AI within that.
For 99.99% of cases, you're reading and writing this under an operating system whose kernel is written in a language without send/sync, and inside a browser that also largely written in languages without send/sync, because those systems are fundamentally well designed. So instead of fixing the bugs and rethinking the architecture, the author of Bun decided to transpile almost the entire codebase from Zig to Rust without a deep architectural review. Okay...