Correct, diabetes is related to insulin resistance. However, one can be insulin resistant and not have diabetes.
Saying insulin resistance due to fat intake is the mechanism of T2DM would be like saying the mechanism of car crashes is slippery roads.
One can drive on slippery roads and never crash, therefore a slippery road is not the mechanism of car crashes.
Likewise, one can get T2DM without fat in their diet, therefore dietary fat is not the mechanism of T2DM.
Your disbelief at above only further cements your lack of understanding of the physiology.
How do we T2DM?
1. Chronically elevated BGL desensitises cells to insulin
-> this causes an increased needed amount of insulin to produce the same effect.
2. More required insulin means beta cells work harder and fatigue
3. As beta cells fatigue they produce less insulin, which increases BGL, which causes beta cells to need to work harder, which fatigues the beta cells, which increases BGL, which causes beta cells to need to work harder, which fatigues beta cells, which increases BGL, which forces beta cells to work harder (repeat ad infinitum).
-> additionally, high BGL is toxic to beta cells.
At no point is fat required in that vicious cycle, but deposits do exacerbate the situation as fat can hinder insulin and increase circulating BGL indirectly.
As we can see, the primary mechanism is related to the relationship between beta cells, circulating blood glucose, and insulin.
Insulin resistance (caused by many factors, not just fat deposits) is a player, but it is not the primary mechanism.
Basically, you are partly correct but partly wrong and thats the premise of this whole talk I'm giving you.
I really can't make it simpler than this, if you still don't understand I recommend you take an intro to nutrition class or investigate some medical journals.
It is interesting & Veeky Forums related so I do hope you look into it.