I think you are entirely missing the author's point. The author is generalizing from the specific technicalities of C/Rust/etc UB, to the problem with UB which is that should it be triggered, then you can't know what the program will do. This does not have to be the result of language specification. If writing safe Rust yourself, yes no UB will occur usually, and you can know what will happen based off of what code you wrote. The author extends UB to vibecoding where there is no specification to understand the translation of prompts to code. Without thorough review, you are unable to be sure that the output code matches the intent of your prompting, which is analagous to writing code with UB. The issue the author has with vibecoded Rust is not that the code can trigger undefined behavior at the language layer, but that the perfectly "safe" code generated may not at all match the intended semantics.