Your understanding is close, but not quite correct. The thing that splints open your airway is the pressure, not the airflow. Look at it this way. If you didn't need to breathe, but just needed to keep your airway open, you could use a setup without any intentional leaks, and the machine would pump just enough air to create the required pressure, sort of like filling your car tires. And that pressure would keep the tissues that would otherwise sag and block your airway back in check.
Unfortunately for the engineering, you also need to breathe. Meaning, you need to take some of that air into your lungs when you inhale, and then add back some more air from your lungs when you exhale. Now, without any intentional leaks, the machine could still cope by pumping air into the system when you inhale to keep the pressure up, and sucking it back out when you exhale to keep the pressure down. But this means that you would be constantly breathing back your exhaled air. Not good.
Instead what they do is introduce intentional leaks into the system. This keeps the air flowing, making sure that you always have fresh air to inhale. Now, imagine how it would work if you didn't need to breathe. Just like a car tire with a slow leak, the machine can keep the pressure at the specified level by constantly pumping air in at the same rate as it is leaking out. So now, when you breathe, the machine just has to pump less air when you exhale (because you are supplying some of that leakage flow), and has to pump extra air when you inhale to compensate for what you are inhaling.
That is why unintentional mask leaks are not necessarily a problem. They are just extra leakage that the machine has to compensate for. There are only two requirements on the leak rate. Firstly, obviously the machine has to be capable of pumping the airflow equal to the total leakage rate plus your inhale rate. Secondly, the machine needs to be able to determine when you inhale and exhale. Ignore the machine airflow capability for a moment and imagine a HUGE leak rate. If your breathing is a small fraction of that leak, it is statistically difficult to determine when you are inhaling and exhaling, because the pressure sensors have a certain accuracy and precision. Hence, the leak rate must be small enough that the machine can statistically determine your breathing pattern given real-life sensors and their associated errors.
Does that clear things up for you?
As a secondary function, don't the vent holes also prevent suffocation if the power goes out? At least with a full face mask?