Ah, but gravity does matter with canisters!
Try priming a can that is above or at tank level... Its a pita!
As I said above, the "gravity doesn't matter" statement refers to the fact that the two siphons (intake and return tubes) precisely cancel each other when completely filled with water
. I.e. gravity doesn't matter when the system is completely filled.
For example, if you unplug the pump in a filled and operational canister filter, the water will just stop. It won't flow anywhere. (Compare that to a truly gravity-fed system: a wet-dry filter. If you unplug the pump, the intake siphon will continue to feed water, eventually overflowing the filter.)
When the canister filter is empty and you just begin to prime it, then, of course, gravity does matter.
If the canister is located below the water level in the tank, then it can basically prime itself - you just have to give it a few pumps to pull the water over the level of the tank. The intake siphon will do the rest (although the hand pump might do it faster).
If the canister is located above the water level in the tank, then, as you correctly noted, you will have to keep pumping pretty hard until you fill the whole canister and the return tube.