The Chinese 996 work schedule is indeed hard work but it doesn't mean working 72 hours according to how westerners calculate working hours. For one, tech workers arrive at the office at 9am, not that they start working at 9am: once they arrive they simply head to the office cafeteria to have breakfast between 9am and 10am. And it's really common to spend one hour each for breakfast, lunch, and dinner. Unlike westerners who'd be happy eating some cold sandwiches for lunch, the Chinese are rather picky about their food being hot and served with soup. Second, the Chinese have a weird culture of napping at the office after lunch. Not just some nap pods here and there, but institutionalized naps with day beds for all. So right off the bat, you deduct up to four hours of non-working time from twelve hours of in-office time. Finally, the Chinese also have a culture of not leaving before their bosses leave the office. This means even if a worker has finished all assigned tasks, the worker simply kills time and waits for their boss to leave. It might be reading internal documentation, or it might be something less productive.

996 refers to time in office, not time worked. The actual hours worked is closer to 48 hours per week than 72 hours.