The science makes sense. Fish physiology and fertilizer exposure and such.It is science, that's how fish physiology functions. Think of it like a person standing in a closed garage and breathing in exhaust fumes; to think the air in the lungs will not allow this to get into the bloodstream is false. Any substance added to the aquarium water similarly enters the fish.
The substrate fert was Nutrafin's Plant Gro Sticks; they came in a pack of five, and they were effective for a full year, and boy did the sword plants thrive. I have not been able to track them down online for 9 or 10 years now. I suppose I could write to Nutrafin, but I tried the Seachem Flourish Tabs and they are very good too, so I haven't bothered.
Education is good and that's appreciated.
However, I detect some ethical judgement here regarding those who choose to use liquid fertilizer in the presence of fish. That is, actually, not scientifically motivated. Might want to dial that "right/wrong" stance down a bit and stick to the facts. Assuming the science is there and liquid ferts are harmful to physiology of fish, the science should be all that's needed.