I don't know how to articulate the idea any better. Our closed loop systems are still exposed to atmosphere on both ends, so basically you've got a siphon tube, albeit a modified one, being the presence of a pump creating pressure and velocity of the fluid. We agree that if the pump were to stop, and velocity were to stop, and the loop being high enough, your max siphon height will see a limit.
Now, let's make our hypothetical loop 100' vertical initially filled with water, install an absolute pressure gauge (properly) at the peak of the loop, at the beginning of the upboud side (after the pump), and ending of the down bound side, slowly ramp up pump rpm. What do they read as impeller rpm transitions from 0 rpm to 2000 rpm? What is in the crest of the loop (water or water vapor) and at what rpm?
Sheer forces within the boundary layer (restriction) are on our side this time.