That's no hard proof, there's a correlation between people who consume lots of non-sugar sweeteners and diabetes. This could very well mean that people who are prone to eating lots of non-sweetener sugars are also prone to eating just plain sugary foods. A few causal mechanisms have been proposed, like non-sugar sweeteners making your glucose spike but there's really no evidence to support this. The new advice from the WHO makes sense in this aspect, switching to non-sugar sweeteners is not enough to lose weight or avoid diabetes, people need to consume things that are less sweet in general. It also explains why the new recommendations don't extend to patients who already have diabetes. It just strikes me that switching from normal coke to diet coke is a poor way to avoid diabetes.