What I do like about these blocks is size of the pores. I have noticed quite a few worms going in and out of them and pods crawling all over it. I don't believe sintered glass has that?
Perhaps too large to allow an effective anoxic zone to be created to facilitate de-nitrification?
In my experience, I had an 8x8x4 block in my sump, & an ATS running. The nitrates were fixed at 5 to 10 ppm.
I removed the block from the system, leaving just the ATS, & the nitrates fell to zero in a week.
In my case the block was acting as a nitrate factory.
On the box it comes in it does state - "Designed specifically to target ammonia and nitrite removal and minimizes nitrates."