It's probably the phenethylamine
I think the more responsible null hypothesis should be that people respond to diet coke for the same reason they respond to all caffeinated drinks, because of the caffeine.
The only real way to test the hypothesis would be for non-caffeine drinkers to drink decaf diet coke. Without that, any investigation is going to be confounded by the 46mg of dopaminergic stimulant in each can.
You have missed the elephant in the room, its the methanol.