I'm currently using Seachem Matrix in my canister filter which is the same thing but for higher flow rates, although I don't have any particular data to verify its effectiveness or whether the flow rate there is still too great for it to work optimally.
Deep within the porous structure of the granules, the water flow is supposed to be slow enough to allow anerobic bacteria to denitrify the water. Basically, the anerobic bacteria are equipped to be able to strip the oxygen from the nitrates in order to provide for their own respiration and survival, with the byproduct being harmless nitrogen gas which just gasses out of the aquarium then. It's the same reason reef aquarists have been using live rock in their tanks for years, as anerobic bacteria do reside within it.
I do still have some level of nitrates in my water so it's not depleting them entirely, although I can't tell at what level due to my test kit being rubbish. And I'm not sure how much of the nitrates produced are being absorbed by my plants versus the Matrix. The plants do seem healthy but I haven't seen a great deal of new growth, so I may be limited on some nutrient being that I'm not dosing much of anything. As such, the Seachem Matrix is at minimum a significant safeguard to ensure that my nitrates don't escalate in case my plants aren't consuming them for any reason.
And theoretically, an aneorobic filter medium such as these would allow you higher fauna stocking densities without excess nitrates resulting, in case your plants alone can't keep up with them. And if your nitrates are getting depleted then just reduce the amount of media that you're using to rebalance things out. My tank is a bit of a trial and is heavily overstocked I must admit, with generous feedings and no water changes. My trial methodology is perhaps a bit different than the usual approach, and seems to be working thus far although that's not long enough to draw conclusions from for its long term effectiveness. And water changes wouldn't help me reduce my nitrates either, being that my tap water has higher nitrates than my tank does...