100% nonsense. Just because he's a Chinese living overseas doesn't mean he knows a wit about what goes on in China. I have multiple stamps from all four of those (Greater China) locations in my U.S. passport, and later this week I will be making my third trip of 2019 to China with no worries that I would be prevented from exiting. It's true that secret police are everywhere in China, physically and electronically, but they don't target the thousands of individuals every day who go back and forth between the Mainland and Greater China locations simply on the basis of having visited those places.