This article suggests an interesting theory, but the author didn't actually test it. He was only sharing something he found elsewhere and even that source seems to only "suggest" what might happen. Personally, I doubt that much, if any, anaerobic bacteria develops (to reduce nitrates) in an UGF because the water movement is unlikely to be slow enough and, I would think, any water moving completely through the substrate is going to be oxygenated. Additionally, you are supposed to deep-vacuum regularly, to keep flow going.
Anecdotally, my experience was that nitrates climbed as a function of the sludge buildup under the UGF. I was able to remove about half that sludge by inserting a flexible tube down the flow tubes and suck out some of it (a difficult process). After doing this, nitrates would drop, but we are only talking about 5-10ppm. As I mentioned, above, I found that by adding a HOB, I was able to reduce sludge buildup, somewhat, and keep nitrates lower but, perhaps, the HOB was simply adding more stabilization to the overall filtering need. I finally bit the bullet and pulled the UGF out. This further reduced nitrates. However, I will admit to habitually overfeeding my overstocked tanks and, as you know, this can contribute to sludge problems.
I'm not sure you looked closely enough and it would seem you didn't bother with the embedded videos. This is not unlike deep sand in SW aquariums.
The objective with a very slow flow rate (of a gallon or two per DAY) is to create an anoxic environment, and not an anaerobic one. With such a low flow rate, deep vacuuming is not required or desired. This is not unlike Dr. Novak's research in anoxic biocenosis clarification baskets, just fits neatly inside the aquarium. Also, he claims to have systems running this way for up to 10 years, with 6 months between partial water changes and 5-10ppm nitrates consistently (although it's lightly stocked and has plants).
So maybe it's nothing, or maybe it's really something!
BTW, I guess you missed that I AM the author of the article reporting the research of Dr. Kevin Novak Ph.D. As I mentioned in the article, I have been testing anoxic biocenosis clarification baskets
, but yes, I have yet to test using the UGF plates as a plenum.