If your tank is so unhealthy for your plants that they stop growing, yeah, at that point they won't be taking up nitrates or CO2. If they are growing at all, they are consuming some, though the amount will be less than if the plants weren't being growth limited.
As with so many things in the hobby, you have to look at your system as a whole. The high nitrates in your water column doesn't mean that your plants aren't consuming any, just that they are being produced at a faster rate than the plants are using. Maybe your plants are growing well, but you have a lot of fish and don't do many water changes. That could result in high nitrates, and it would be more easily remedied with water changes rather than messing with the plants. Plants that have access to the air are sometimes called "nitrate sponges" because they grow so fast they can use a lot of nitrogen.
Some people have a system that doesn't require extra fertilizer, but their plants are getting all their nutrients from other places. These tanks tend to be a specific flavor of tank - no co2 injection, lower light, light fish loads, lots of easy plants. Everything is low demand and slow growing and it's easier to keep in balance.
One day I met a guy who is into aquascaping and he made some really good aquascape with carpet grass and background plants. I asked him which fertilizer, and nutrients formula he is being giving to his plants. He said he as 5 massive sized tank (50 gallon) and for such a large quantity of water it will cost him a fortune for fertilizer and nutrients. He said he let plants get nutrients naturally from fish waste and uneaten food.
If he's found a way to make this work, then that's great, and if you want to follow his method you'll need to copy them as closely as possible. But if you learn to you dry salts fertilizer isn't especially expensive, especially compared to everything else in the hobby. It just requires some research and math.