I think there's two parts to this. First, there's a bit of a history of people making disingenious jabs at Rust for not having an "ISO C++" style spec. Typically people would try to suggest that Rust can't be ready for production or shouldn't receive support in other ecosystems without being certified by some manner of international committee. Second, Rust by now has an extensive tradition of people discussing memory safety invariants, what soundness means, formal models of what is a valid memory access, desirable optimizations, etc, etc, so your question what undefined behavior means could be taken to be, like, polemically reductive or dismissive.
In context I don't think it's what you're doing, but I would also not be surprised if a lot of people reading Rust-related HN discussions are just super tired of anything that even slightly looks like an effort to re-litigate undefined behavior from first principles, because it tends to derail more specific discussions.