If you have a lot of LR, you may not need the canister...but a sump will not solve your problems if the problem is from overfeeding, overstocking, or lack of maintenance. The sump is nothing special. Perhaps they mean a wet/dry, or a refugium. But a "sump" is nothing more than a container with water brought from the tank and then pumped back to the tank...where people tend to put a skimmer, heater, etc. That is all. Not a filter in any way in its pure sense. So no, I would not tell you simply to replace the canister with a sump because they are apples and oranges in many ways. A sump is not a filter.
A refugium will help nitrates, a skimmer will help nitrates...both of these can be associated with a sump. But a sump alone does nothing, IMO, to address a nitrate problem.