As I understand diet coke uses sucralose, not aspartame as sweetener.
Sucralose has a different sweetness profile, much closer to real sugar and is not bitter. Compared to aspartame in zero/light that needs 0.2g salt/liter to cover up the bitterness.
I see that the study uses 48mg sucralose in 60ml water amounting to 0.8g/liter.
My Cola recipe with pure sucralose uses 0.167g/liter this should be equivalent to normal soda sweetness of 100g sugar/liter.
The issue I had with sucralose was that it seemed to go out of solution in the syrup that's why my current Cola recipe uses a mix of sucralose/saccharin/stevia in the ratio of 30/30/40 which amounts to 0.05g sucralose/liter so if I drink 1 liter of soda I'll get the same amount as their study.
The reason I continue to use sucralose is that it rounds out the flavor of saccharin and stevia.