That sounds like an implementation difference, not an architectural difference. If they wanted to, what would prevent Node or a third party from implementing parts of the stdlib in a faster language?
The key architectural difference is that Node.js implements the HTTP stack and other low level libraries in JavaScript, which gives it memory safety guarantees provided by the v8 runtime, while Bun/uWebsockets are a zig/C++ implementation. for Node.js, which is focused on enterprise adoption, the lower performance JS approach better aligns with the security profile of their enterprise adoption target.