alt Hacker NewsBoth are wrong. It means "this standard does not constrain the behaviour of code that does this".
It's entirely legal for implementations to have predictable behaviour, documented or not, for code that is undefined by the standard. In their quest for maxxing benchmark performance they generally choose not to, but there's really nothing in any standard that stops you from making an implementation that prioritises safety.
Every implementation so far has predictable behavior in all cases. Sometimes the rules for predicting it are very obscure. But it's all fully defined within the compiler's binary code. And none of them link to nasal portals.