Since you said the NPK numbers on the product are 0-0-4 it's pretty simple to see that it seems that it doesn't have any nitrate, phosphate, and only 4% potassium so it seems to me like maybe the guy has no clue what he's talking about.
Seachem Flourish Comp. contains every nutrient needed for plants, but it's meant to be used in conjunction with the other Flourish products like potassium, phosphate, etc....
This is why 80% of planted tank owners eventually move on to dry fertilizers because it's to manage the amount of each fertilizer, but it also let's us treat deficiencies easier because we can target the deficiency without having to add a bunch of unnecessary nutrients to the tank.
15g Fluval Flex - Will be a planted shrimp tank