Probably an experiment due to Bun's PRs to Zig being rejected (Zig does not allow AI use). If Rust works well enough, and the alternative is maintaining a fork of Zig, I'd guess they'd go with Rust.
Also, if Zig itself doesn’t accept AI contributions, it’s probably NGMI unless somebody is willing to maintain that fork.
The anti-AI policy had nothing to do with Bun's PRs being rejected. This post[0] by a core zig maintainer explains why the PRs were low quality and subsequently rejected.
[0] https://ziggit.dev/t/bun-s-zig-fork-got-4x-faster-compilatio...