Yup, you're wrong. This keeps coming up for some reason, don't know why it's very basic, anyway... Because it's a sealed loop all the pump has to do is move the water any distance the outlet is above the water level. Take a hose, put one end in the water, suck on it until you get a siphon going then dangle the center of the hose near the floor and hold the other end up above water level. If the pump had to pump the water back up to the tank's water level then when you do this the water would simply stop at the lowest point in the hose because there is no pump to pump it back upwards. Obviously, this isn't the case. The water naturally flows through the hose until it's matching the level in the tank. That's exactly what it does in the cannister filter. Now if you lift the end of the hose up so it's say 6 inches above the water level, that's the only height that the filter would have to actually pump it. So, you're only losing 6 inches of head worth of flow and whatever resistance there is in the hoses and inside the cannister. Drop the outlet into the water and then the only losses are resistance in the hoses and cannister because there is no head.
Now if you were talking about a sump, then yeah, that's not a closed loop and the pump WOULD have to pump it the height up to the tank.