This instruction video isn't correct. The fishfood should cover the needs of the fish as accurate as possible. Shifting to a lower protein diet (sub-optimal) is going to result in more waste, lower food utilization % (higher FCR), and maybe even compromised fish health, thus this makes no sense to me. IMO better would be lower total feed load, less fish or more plants and/ or secondary nitrate removal. Water replace, dnf etc..
Btw if you stop feeding the organic waste in your system will still be broken down slowly, further bacteria mass will go down too, again with the release of nitrate, thus there can be a long delay in reduction of nitrate production after reducing input.