OP - From the standpoint of water changes and nitrate control, there's some amount of subtlety to the answer. Typically, when one sets up a new tank with all dry rock/sand, one adds either an inorganic source of ammonia and bottled bacteria, or an organic source of nutrients such as a raw shrimp/fish food, or some combination to promote the establishment of nitrification bacteria.
The typical result of this procedure is relatively high nitrate levels after the 4-6 weeks of cycling, and the general recommendation is to change 1/2 or more of the tank water to bring these levels down. On a purely mathematical basis, the reduction in nitrate is in direct correlation to the amount of water changed - if you change 1/2 of the water volume with new saltwater, the nitrate in the tank water will fall by 1/2.
The reason that you might hear that water changes aren't the best method for reducing nitrate is a slightly different situation. Specifically, the advice is given for an established tank with a fairly high amount of livestock. In such a situation, the tank inhabitant's metabolic processes together with nitrifying bacteria are working to continuously produce nitrate. Depending on how dense the livestock is in the tank, how much the aquarist is feeding, etc..., that nitrate production can be rather high, and one would have to do an unreasonable amount of water changes to control the levels.
Instead, one typically employs methods to reduce the production of nitrate from the decay of uneaten food, and removal of nitrate by harvesting organisms that use it. Examples of the former are mechanical filtration methods such as reef socks, roller fleece and skimming, and examples of the latter are harvesting of algae from a refugium, skimming to remove bacteria from the water, etc... In fact, carbon dosing uses exactly this strategy - one adds an inorganic, easily oxidizable carbon source to the water to promote the growth of aerobic bacteria that are then skimmed out, and to promote the establishment and growth of anaerobic autotrophic bacteria that reduce nitrate to nitrogen gas.
