This assumes that deciding is possible. And to do that, to decide, to choose among options, one must already posit free will. In effect you would be saying that the freedom do decide must be free, which is kind of a tautology
Its an observation of the thinking process, not an argument from logic.
Whenever we try apply logical analysis to observations, it appears as tautology.
Why? Just because everything that happens, including our thoughts, is, in a sense, predetermine in no way insures we know them.
My point being, that IF they were totally predetermined we would not have decisions.
Consider a path that diverges in a fork.
We can always try and look a little bit further down each of the two paths,
but we never can see all the way down.
Somewhere we have to decide to stop looking down the two paths
and make a decision based on limited information as to which way to go.
We can always try and look a bit further down both.
Get binoculars, a telescope, or wait for someone else to come past.
We could procrastinate forever, the longer we wait, the more information we gather.
By analogy, we can always think about two options forever.
We are largely free to think or not to think.
But somewhere, we just decide to stop,
and we take a path.
We never have all the info, so we can never be 100% certain.
If we were 100% certain, we would have no decision,
we would be unfree.