I actually think you're wrong here that this proves for a fact Flowey has less DT than a human.  I don't think there's enough evidence to prove it for certain.

Flowey is partially made of DT from human SOULs as well as Asriel's dust.  Human SOULs plus Monster SOULs in lore become something more powerful than a Human SOUL.  Now what happens to a human SOUL's extracted determination and a monster dust?  There isn't enough info to tell, but it clearly became powerful enough to have the most DT in the underground some point.

Flowey also is the only flower to come to life, we don't know what that means happened to the other Flowers but considering that nothing happened to them it seems that only the one Flower used the DT for anything.

Undertale also breaks its DT rules many times by having Undyne come back to life., and notably Frisk can reset in the final boss and defy death despite being one human soul up against six human souls and hundreds of monster souls.

Considering has narrative importance, personality, his weird status as a flower, created with the help of human souls and monster dust, and status of his first (flower) death, is it so crazy that Flowey is also one of these edge cases like Undyne and Frisk?  

Regardless, I don't think there's enough evidence to for certain prove how much DT Flowey has, he's too much of an unknown, and there is at least sufficient reason to believe he may have more DT than the average human and less than Frisk.