"constantly" can't be done for a certain time-span. You can have 64 "move" actions within one trigger, and those are indeed one after another without wait, but a preserved, hyper-triggered move command does not move the unit with a 0ms wait. Come to think of it, you would move the unit infinitely often within 1 ms.
Instead, the move is executed 12 times per second. Still the unit gets somehow invincible
Must be something in SCs engine that a unit counts as "out of range" as soon as it is moved by a trigger.