Memory Model in C++ : sequential consistency and atomicity

Question

I have some questions connected with memory model in C++11.

On https://www.think-cell.com/en/career/talks/pdf/think-cell_talk_memorymodel.pdf on the 29. slide is wri

Answer 1

The only possible way to define semantics for a language with undefined behavior is by describing sequential execution. Ergo, all programs behave as with sequential execution, or no program has defined behavior.

The idea that you have both in C/C++ is a hoax. Programs must behave sequentially for the semantics to make sense.