There is no reason to think that.
It is at the level of thinking we are nothing but a brain in a vat somewhere. Nothing even suggests it is true.
WE know what movement without the mind causing it feels like. It is like the patellar reflex. The leg just jumps on it's own.
That is not the way voluntary movement feels. The way voluntary movement feels is like the mind is in control and commanding a specific movement and the movement occurs exactly as desired.
If the brain is moving the body on it's own without the direction of the mind why does the mind experience being in control? Why this wasted energy?
It takes energy to trick the mind and make it believe it is in control.
I told you that I don't have a positive position on free will. We may have it or may not. Your certainty is premature.
If you think you have the ability to make the decision you already believe you have the freedom to do it.
What decision do you think you do not have the power to decide?
Can you decide what time you will go to bed?
Can you decide what you will drink?