Its quite noticable that a discus poop decomposes much faster than a goldfish poop . Keeping goldfish through the years i have notice that their poop can stand on the bottom much longer in the original shape and state as opposed to discus poop .
So having this in mind , maybe the video does holds some thruth about goldfish poop not causing instant pressure on the biosystem , since it can be siphoned out before it starts decomposing and fouling the water.