> "ISO C does not say anything about write(2), hence using write(2) is undefined behavior according to the C standard", which seems like a rather tautological and useless statement to me.
That is actually not so useless at all: if you try to compile and link a program that declares and calls a function but does not define it, you will typically get a linker error about an unresolved reference. If the name matches a non-ISO C library function, however, the implementation cannot know whether your program is in error or whether you want to use that library function, and will usually accept it. For this reason, the C standard does actually make it clear that using write(2) is UB to make it clear that implementations are not required to diagnose that as an error.