The author seems to be missing this essential text: "the implementor may augment the language by providing a definition of the officially undefined behavior."
Making a system call is undefined behavior in the C standard, but it's not undefined behavior in clang-on-FreeBSD, because the implementors of clang on FreeBSD have defined what those system calls do.
Ditto for "asm" (UD unless/until you're running on a compiler which defines what that does), all of the tricks which make "malloc" work, and all of his other examples of acceptable uses of code which the C standard does not define.
Making a system call is undefined behavior in the C standard, but it's not undefined behavior in clang-on-FreeBSD, because the implementors of clang on FreeBSD have defined what those system calls do.
Ditto for "asm" (UD unless/until you're running on a compiler which defines what that does), all of the tricks which make "malloc" work, and all of his other examples of acceptable uses of code which the C standard does not define.