The notion of moral responsibility presupposes that we can do what we 'ought' to do. The argument from moral necessity does not prove the existence of free will. It simply asserts the requirement to believe in free will if we assume the existence of a moral context. If we reject the notion of morality there is no reason to acknowledge the existence of free will. The compatibility thesis argues that both views are correct but entirely compatible with each other.
Determinism is correct in that every event has a cause.
Yet there is a difference between voluntary action and involuntary action. The compatibility thesis proposes that some events are, more or less, free, while others are constrained by external causal factors. The compatibility thesis in this form does not allow for free will but does acknowledge that some actions are free while others are not. A development of the compatibility thesis. They are 'internal' to the reasoning and choosing of the agent.