It's not the actual RPM that kills them, its the rapid change in RPM. I.E if my stock truck hits 5000rpm but takes awhile to get there vs my built truck just snapping the tires loose to get to 5000rpm instantly. It is that inertia that kills the pump.
If the coolant is already moving at a high velocity its doesn't hurt it but trying to move an incompressible fluid from slow to high velocity very quickly just doesn't work and puts alot of stress on the press fit/weld/pin etc.
That said, I believe the fins on the plastic impeller "deform" releasing some of the coolant but that is just a theory.