The difference is that in preparation to fire a rocket, you only need to apply ullage acceleration for a short time because after the rocket lights, that provid

The difference is that in preparation to fire a rocket, you only need to apply ullage acceleration for a short time because after the rocket lights, that provides the acceleration. When just transferring fluid, you need to accelerate during the entire period of the transfer, because pumping fuel to another tank provides no acceleration of its own. So, you need the acceleration to be small enough not to build up to an inconvenient delta-V over the time it takes to move the fluid. In principle you could apply acceleration for a while in one direction; and then switch to accelerating in the other direction, wait for the fluid to settle at the other end, and pump from that end, alternating back and forth. But it still consumes reaction mass, and introduces delays while switching. Docking vehicles massing hundreds of tons is a much bigger problem than docking Dragon. The Dragon experience might not apply very directly. Presumably the engineers actually involved all understand this (literally rocket science) much better than we do.