No. Undefined behavior means one thing and only one thing, at least when it comes to C: "behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements"

There's no such thing as "insofar as both specifications apply to it, that is defined behavior, not undefined behavior." Undefined behavior in C that is standardized by POSIX is still undefined behavior. Indeed, that is why "Possible undefined behavior [includes] behaving during translation or program execution in a documented manner characteristic of the environment." Behaving according to the POSIX specification on a POSIX system (or according to the specification of any system on that system) is explicitly accounted for in the definition of "undefined behavior."

These things are objectively, inarguably true, and recorded in black and white. I do not understand why you are so relentless in trying to gaslight HN about them. Just stop.