What I've read, the theory is that matrix is porous enough to create 'pockets' where anaerobic bacteria (some of which will consume nitrates) can grow. On the other hand, the bacteria you need to break down ammonia to nitrite, and then nitrite to nitrate, very much require oxygen.
So...the question is how effective Matrix really is at providing a place for both aerobic and anaerobic bacteria to grow? I don't know from firsthand experience.
I wouldn't use Matrix as a substitute for water changes, since evaporation etc will contribute to higher and higher levels of total dissolved solids (TDS) in the water. So you still need water changes to get rid of those dissolved solids...and they can also get rid of nitrates.
So it's basically a balance between frequency of water changes, density of stocking, helpful aids like plants that will (or media that may) consume nitrates. There are several ways to do it.
Personally, I don't find the added cost of Matrix to be worth it...but that's just me. Perhaps if I tried it I would find differently.