I have seen claims like this where using soil as substrate, or the use of clay cat litter in a thick layer under normal substrate in nitrate detoxification.
Whether or not this is possible when dealing with large fish like cichlids, or other diggers is debatable.
Most arguably successful claims are made with smaller species, that don't dig up the substrate, compromising the anoxic redux potential that is required for it to work.
And
the size fish used are usually species with an average lifespan of about 3 years, many less, some slightly more.
Many Tetras and live bearers fit into this category, and basically kick the bucket before the delirious effects of elevated nitrate appear.
The chronic effects of elevated nitrate on fish like cichlids, don't often present until the fish is about 3 years old, and as most cichlid keepers know,
many cichlids have a lifespan, of 8 to well over 10 years if kept right, but if kept at higher nitrate concentration look like they have been in a continuous bar fight, only the truth is, they are scarred up by HITH.
And people that claim success with experience after 6 months or even a year with an oscar, or other cichlids, I consider delusional.
Only after 3 to 5 years with a cichlid, are you just breaking the surface, of true experience.
And its not just nitrate we are measuring when we measure nitrates, because nitrate is an indicator chemical species of pollution, , in the same way use use coliform bacteria in microbiology to determine drinking water with fecal contamination.
We do not have the capacity to test for all the other pollutants, so we take measurements of nitrate to determine the soup of contamination in a tank