Here is a fact: every event has a cause. This fact is known as determinism.A little internet searching and one can find a few others defining determinism in this way. But that definition seems to me odd. For one, simply having a cause is not sufficient for an event to be determined, for it is broadly logically possible that an event might have a contributory cause but no necessitating cause.
The common definition of determinism is not that every event has a cause, rather it is the following: For any event or state of affairs, given some event E or state of affairs S at time T1 and the laws of nature, another event E2 or S2 must (nomologically) occur or (in the case of states of affairs) obtain.
This definition (or something close to it) is what compatibilists have in mind when they say that free will is compatible with determinism. This definition also leaves open that the first event (or initial state of affairs) has no cause, which Conee and Sider's definition does not, and so has the virtue of being consistent with what many determinists actually believe, namely that determinism holds in our universe even if there is no cause of the first event.
Their statement above is also problematic because of the claim that determinism is a known fact. Kant spends a good deal of the Critique of Pure Reason trying to establish that every event has a cause (as do others recently such as Alexander Pruss in The Principle of Sufficient Reason.) If it is a known fact (I think it is a fact) it is certainly not the sort of known fact to be included in an introductory philosophical work stated as a known fact. Especially when it is stated as determinism. Determinism, properly defined, is certainly not a known fact.