This topic is one of many that is difficult to really understand if we want scientific information. The science is out there, but there are many more claims that are not based on science, so you really have to look for it. And when you do find it, you have to be able to interpret it correctly.
I teach college classes on research design and 2 things I always teach my students to look for are: 1) What's the sample being studied and 2) how did they measure what they're looking at?.
Regarding samples, Kaplods point that humans aren't rats is correct! We cannot necessarily conclude that what happens to them generalizes to us. Clinical trials for medicine used to only be conducted with men, so do those results apply to women. Are obese people different from non-obese? The research is out there, but there may not be enough to conclude it applies to a specific group.
As for the measurement, it is a broad question -- did they just look at one sweetener - doest that apply to others? They studied diet sodas, but what about other foods??
Science is the only way to answer a number of questions, especially causal questions. And those causal questions can only be answered by experiments (much of the research is correlational) But every single study is very limited, by it's nature, and we have to be careful to build a correct picture.
All of that said, I once read that there is a lot of conflicting evidence about the value of a number of foods on diseases, e.g., red wine, coffee. Some studies find positive outcomes, others negative.
But diet soda didn't seem to have any 'positive' outcomes regarding diseases. In other words, diet soda may not harm you, but other than helping to reduce calorie intake, it doesn't seem to provide any other benefits (e.g., heart disease, cancer). So as a consumer, you can think about whether the calorie decrease is enough for you given the possible health issues. Personally, I have decreased the diet soda I drink, but have not given it up entirely.
My 5 C's of healthy living: Commitment to conscious control, with the understanding that choices have consequences