What are you talking about? There is no upstream rejecting contributions here. It's the original bun developers who vibe-ported it to rust and they absolutely could and did upstream their vibe coded changes because they are the upstream.
https://ziggit.dev/t/bun-s-zig-fork-got-4x-faster-compilatio...
Bun were so excited about their 4x speed improvement that they missed that Zig had already implemented it, plus other optimisations that were far larger.
Nothing in your link supports this assertion. Lots of things in your link support the exact opposite in fact, that the Bun team explicitly chose not to attempt to upstream the changes.
They may have been rejected for being vibe coded if they were original, but they were rejected for being pointless. The rust rewrite was because Bun was butt hurt that they didn't actually help.