A lot of autistics have indigestion and/or food intolerances.
I gave up soda and as a result, my skin stopped itching and cleared up. I was afraid I was prediabetic, but having blood tests taken, the doctor confirmed this was not the case.
However, refined sugar is bad for you. It causes an insulin spike which you get addicted to. Then you crash and need more and you end up more tired than you were before ingesting the sugar. Eventually, if you keep a high intake of sugar over a prolonged period of time, you mess up your pancreas' insulin production and that's the road towards diabetes.
There is also High Fructose Corn Syrup and Carbon Monoxide in soda, which is not easy to digest.
The only sugar I eat now is fructose naturally from fruits. It was difficult to cut out refined sugar, but it was worth it. It became easier as I started exercising regularly. If you're in very bad shape, it's almost impossible to improve your diet, because you get energy from insulin spikes and treat depression with bad calories and sodium. I'd know, I was addicted to sugary sweets, sodas and junk food for years.
There is no question if soda is bad for you or not. It is. The question is, is it worse for autistics than other people ? Only if there is a correlation between autism and indigestion/food intolerance. This has not been proven scientifically I think, but the two seem to go hand in hand. Most autistics I've talked to have food intolerances, but then again it is a rising epidemic among all the population of the west where we eat junk foods and sugar and exercise very little.