As far as I'm concerned it was a number of years eating too many carbs that caused me to end up overweight and eventually with a T2 diagnosis. By cutting those carbs right back down, within 3 months I ended up with pre-diabetic glucose levels, and - without actually doing anything else- I also lost 20 kilos in weight..
I grew up eating a normal/full fat, moderate carbs diet, and didn't change my fat consumption at all either before or after losing weight - but cutting carbs enabled me to burn off some of my surplus stored body fat as fuel instead. I currently eat below 50g carbs a day and slightly more fatty food thna previously, my glucose has remained at mainly low-end pre-diabetic levels, and I recently lost another 10 so so kilos after my reduced weight was stable for many years .
So for me, I have no doubts at all that scoffing too many carbs was the cause of both my extra weight and my T2 diagnosis.