They replenish the soil now and then. They do have 1 or 2 tanks (older ones from what I saw when I visited) that has more serious dust on rocks. The rest of them are pretty clean. And also they can just replace plants that are not doing well - so pictures can be deceiving, they remove types of plants that are not doing well over time. ADA seem to have updated their fertilizer range (I'm not updated on that, but I think they have NO3 dosing now... not sure in what amounts).
Tank cleanliness plays a large role. My farm tank has 8 Tubes of T5 over a 47gallon - extrapolating from PAR tables that is nearer to 200 umols at the substrate than 100... that tank is dosed quite lean as well but just as spotless. Vin kutty came and saw that setup for himself - and we pretty much concluded that its more the effect of consistent plant upkeep (pruning, replanting) & maintenance (vacuum substrate now and then), consistency/quality of water parameters, biological stability of a matured tank - things that can be definitely replicated elsewhere with "some training and experience".
I have worked the last week or two on meticulous cleaning of my tank. Water changes every 3 days 50%+, weekly cleaning of the filter, , and comprehensive blow and vacuum of all plans and the substrate during WC. (thanks for the new method!)
Before I had a Chihiros A601 and running it at 100% gave me algae very quickly(within hours GDA on hardscape). GDA, GSA, and HA. I could get GDA on hardscape and glass within a couple of hours if I ran it at 100 par at substrate 150+ on some hardscape. Had to run it at 60% and get 50 - 60 par at the substrate.
Now running a Twinstar 600S at 70% which is the same/greater PAR(80 - 120 at substrate) or a bit higher, better spread, and I haven't seen any GDA or GSA in a while.
Front glass is pretty clean I haven't had to clean it in days. I have never had less biomass in the tank as well as I did major trims as well.
Definitely seeing cleanliness being more important than adjusting ferts and even than adjusting light.
I'll be tearing down this tank, your Diorama scape has inspired me to try something similar in an an ADA 60p clone but certainly this thread was helpful thanks. Trying hardscape with lava rocks and spiderwood.