It's a common misconception, but needing insulin does not mean the diabetes is Type I (in people or cats). Many Type 2 diabetics require insulin - either from the beginning, or as a progression of the disease.
This is a bit of a general overview, but basically, in Type 1, the beta cells of the pancreas no longer make insulin because the body’s immune system has attacked and destroyed them. There is no chance of coming off insulin with Type 1.
Type 2 is often caused by insulin resistance, where the body cannot properly use the insulin that is being produced. (i.e. there is usually insulin production occurring, but it is not getting into the cells.) Over time, the insulin production can actually decrease. Diet change, losing weight, etc. are generally used as the first line of treatment, but extra insulin may eventually be required. Some studies show that in type 2 diabetes, there is a steady decline of beta cells that adds to the process of increased BGs. With cats, insulin is generally used immediately because the disease is not picked up until the BGs are seriously elevated.
The vast majority of feline diabetics are Type 2 - and that is why being able to come off the insulin can happen when diagnosed and treated promptly. (Unlike humans, cats seem better at actually regenerating/repairing beta cells).
Hope that helps.