I'm not sure why you observed what you did, but in general, I would not expect large phosphate reductions to follow (especially long term) from the use of Marinepure blocks.
Nitrifying bacteria convert ammonia into nitrate (via nitrite). Like any living organism, if they increase tissue mass they will take up phosphate somehow. But adding a substrate for bacteria is unlikely to alter the tissue mass of nitrifying bacteria noticeably since in reef tanks it is already running pretty much fulls team ahead (since there is rarely any appreciable excess of ammonia). I do agree that if the role is to reduce live rock amounts, this sort of product will serve that need for a nitrifying bacteria substrate.
Denitrifying bacteria convert nitrate into N2. Like nitrifiers, if they increase tissue mass they will take up phosphate somehow. But they use disproportionally more N than P by a wide margin since they are using nitrate both as a source of N atoms for their tissues, and also as an electron acceptor in place of O2, and that extra N disappears as N2.
So when denitrification is an important process (either with something like these blocks, which use natural organics in the water, or when the process is driven by adding organic carbon), the system is often left with extra phosphate that folks usually have to deal with other ways. That is why products like GFO are often used along with organic carbon dosing.
If you read through the MarinePure Cermedia product literature, they focus exclusively on the nitrogen cycle bacteria mentioned above, without mention of phosphate that I have seen.
http://www.cermedia.com/marinepure-technical.php
