No. If we accept your argument then this would be correct:

> In absence of a standard effectively every language construct is 'implementation defined behaviour'.

But the parent said:

> In absence of a standard effectively every language construct is 'undefined behavior'.

Undefined behaviour != implementation defined behaviour. Both of these things exist in C and are separate. You can rely on implementation defined behaviour giving some consistent result on a given, compiler and hardware. You cannot rely on the behaviour of your program if you hit undefined behaviour.