Here's what is buried a bit in the text:

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Something like 95% of vinext is pure Vite. The routing, the module shims, the SSR pipeline, the RSC integration: none of it is Cloudflare-specific.

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The real achievement is human-built Vite (and it is an amazing project).

Since Next.js's API surface and capabilities are known, this is actually quite a good use of AI: re-implement some functionality using a different framework/language/approach. They work rather well with that.