Recent WHO recommendations say that artificial sweeteners are not useful for weight loss, but used in moderation they are not terribly unsafe as far as current studies indicate. All the stuff saying artificial sweeteners are super scary and bad is just that, scare tactics. Or it takes a gigantic amount to be bad for you, but if you replace that amount of artificial sweetener with sugar then the sugar is just as bad or worse. Better just to avoid both.

I try to avoid single studies or articles about anything, but rather look to larger recommendations from WHO or other agencies that are less likely to be influenced by $$$. Looking at a single article or study is basically meaningless. Unless you're Joe Rogan and you're paid to sell everyone a meat eater diet or Dr. Oz with whatever his garbage of the month is.

Is it the diet Coke or the lifestyle of the person who consumes it in significant quantities?

I've noticed that during healthier times of my life when I'm experience less stress, I have no interest in junk food or soda. When I get stressed and I'm not sleeping as well, I get more cravings and find myself making unhealthier choices for the little satisfying bump a dose of sweet/salty/fatty snack and a caffeinated soda provide. Obviously living a stressful life is unhealthy as a baseline.

Could it be that people who are already predisposed to getting overweight try to avoid it by diet drinks, but fail because it's genetics and they take more calories on average? Correlation=/=Causation?