It's one of the common definitions of free will. From AI:
Free will is
the capacity for individuals to make voluntary, independent choices among various courses of action without external coercion or prior deterministic causes.
Note the precise wording: '...without external coercion or deterministic causes.'
From Britanica:
'free will, in
philosophy and
science, the supposed power or capacity of humans to make decisions or perform actions independently of any prior event or state of the universe'
Again: '...independently of any prior event or state of the universe.
From the Cambridge Dictionary:
'...the ability to act and make choices
independent of any outside influence:'
What is presupposes is not the conclusion but one of the premises. As I said in the OP: '...
if existence is deterministic...' I can't prove that it is, just like I can't prove that all swans are white. It's an inductive argument. But you can easily disprove it. Feel free to do so.
Well, if you redefine it as such then you are bound to win your argument. But what you are saying there is that prior events or causes don't necessarily deny free will. So you're saying that something else comprises free will.
Well, off you go and tell us what it is.