I wonder whether even if there is a causal link, the diet version will still be healthier (for low values of healthy) than the sugary version. I tend to think so. I would assume the people most likely to drink diet beverages do so to wean themselves off of sugary drinks. That's one of the first steps in getting towards a healthier lifestyle. The article notes this too. A mystery to me is why they haven't asked about…
I had a coworker who insisted diet sodas were worse for you than regular sodas. I just don't understand how people reach that kind of conclusion.
A random example that does not definitively settle anything but is enough to make you keep asking the question [0].
And one common sense argument is that many people consume way more diet sodas than they would regular ones. Since they have no sugar and almost no calories the assumption is that there is no danger to consume more of them, they are "healthy". Since artificial sweeteners have their own sets of risks in mechanisms that are perhaps even less understood, suddenly this assumption is that more dangerous. It could be jumping from the frying pan into the fire.
[0] https://onlinelibrary.wiley.com/doi/full/10.1038/oby.2008.28...