Soluable fiber slows down the absorption of sugar and improves blood sugar levels but it doesn’t cancel the sugar out. If you eat a ton of sugar with or without fiber it’s still sugar and a diet high on added sugars has many negative affects on our body such as higher blood pressure, inflammation, increased triglyceride levels, diabetes, fatty liver disease etc etc.
You can be normal weight but if your diet has too much sugar still get diabetes and be unhealthy. Recommend listening Peter Attia’s podcast on sugar. https://peterattiamd.com/ama18/