People on the internet keep saying this because they are delusional and it comforts them because they drink regular 24/7 and don't want to acknowledge that it's the cause of their acne and obesity. Not that I'm accusing you of this, of course.
I suppose diet soda isn't "healthier" per se, because neither of them are by any means healthy for you, but it's certainly no where near as bad for you as that high fructose corn syrupy water with food coloring is.