Please link directly to the study (https://link.springer.com/article/10.1007/s11606-011-1968-2) instead of just posting the PR announcement. I'll also note that this is part of the Northern Manhattan Study (http://columbianomas.org/). I'll also note that it's not directly about aspartame but diet sodas in general (so if diet sodas has a consistently higher caffeine, then that could be the reason). I've also noticed this phrase, although the study claims that increased cardiovascular risk on frequent diet soda consumption is still significant despite adjusting to these factors:

Frequent diet soft drink consumption was uniquely associated with white race, former smoking, hypertension, elevated blood sugar, lower HDL, elevated triglycerides, increased waist circumference, BMI, peripheral vascular disease, previous cardiac disease, and the metabolic syndrome. Frequent regular soft drink consumption was uniquely associated with male sex, black race, current smoking, carbohydrate consumption, greater diastolic BP, and lower prevalences of diabetes and hypercholesterolemia.

On the other hand, it was published in 2012 and the study was conducted in America, so almost all non-diet sodas were sweetened with either 100% HFCS or cane sugar/HFCS mix. It's harder to replicate today considering that most non-diet sodas has also added aspartame (or other high-intensity sweeteners) to the sweetener mix.

I'm curious if that accounts for the effects of the caffeine in most diet sodas.