He creates an axiomatic foundation out of definitions that most philosophers of the time would have found reasonable (nowadays they're also reasonable, just takes a bit of work to get past the language choices he made, as mathematical logic didn't exist then so it's phrased in terms of geometric and theological language). He then uses this foundation to show that there's no objective measure for dividing individualness (what is me, what is not me) and responsibility (what was caused by me, what was not caused by me), essentially a proof that given the axioms no such measure can exist. Similar to how one might give a proof that e.g. there is no way to assign a Lebesgue measure to every subset of the real number line. Note this is just a very rough summary; his actual proof is long and dense.

>Isn’t this just free will?

Spinoza essentially shows that if free will is defined as "being 100% the cause of some action", then free will does not exist. Because any action we take, is determined by who we are at that moment, and who we are at that moment depends on actions we took in the past, and this causal chain can be traced backwards to who we were as a baby, when we could not make decisions.

Another way to look at it. If the universe was deterministic, and I had unlimited computing power and storage for simulation, then I could exactly predict someone's actions in life (assuming consistent laws of physics). If it was nondeterministic, then there are some things I couldn't predict, but these things would all be the product of chance, so how could they increase the degree to which any particularly individual is the cause of some action? They would just increase the degree to which randomness was the cause of some actions.