No, they don’t. These are separate concepts with different behavior. “Undefined behavior” is illegal to have in C/C++ programs and includes things like division by zero and out-of-bounds array access. “Implementation specific” behavior is legal but allowed to differ, such as querying the size of an int.

You really don't need to go very far to get into UB instead of implementation defined: signed int overflow is already UB and the program is permitted to do literally anything if you ever accidentally make one of your ints too large.