Water changes are necessary for more than just nitrate removal. They are needed for mineral replenishment, stability etc... Nitrate is just one of the key elements easy to for the common hobbyist to measure, hence the focus point of it. Like saying the warning light on your gauge cluster is lit. In a BMW that can mean something simple. In a Ford it's likely a new engine or vice versa. Similar here when discussing different volume tanks as no two answers would apply to everyone.
Plants do help if you're able to match tank volume/stocking to the amount of plants needed to keep nitrate down. The methods in which plants reduce nitrate is most significantly (not entirely but most significantly) by consuming ammonia first, thus avoiding the nitrogen cycle all together for a large percentage of said waist. Not to say one would have a tank that is not cycled but the size of a bacterial colony will only grow to it's available food source and surface area which a healthy plant filtration would by design, starve out by competition. I really do not like the terms we use in the hobby like they are interchangeable between tank sizes. One of my rays tanks could fill a garage full of plants and I would get little results vs. a drip system like I have today. In turn a drip system on a 55 gallon is crazy, go plants etc... Volume is the key to not only dilution but also the method in which one would go about approaching denitrification (a word I dislike).