The definition of downward causation, in philosophy of mind, is not one that requires a cause to be from elsewhere. It is compatible with emergent states of mind in a physicalist view. I.e. downward causation is a causal relationship from higher levels of a system to lower-level parts of that system. From our perspective as conscious creatures, we can perceive this as mental events acting to cause physical events. (we see higher order activity as mental) It doesn't require an ontologically dualist view that the mental events are other than an emergent process of a physical neural network.
So I agree that on that high-order level where I form intentions, free will can be meaningfully defined as acting according my intentions. To be a compatibilist, if I am understanding that position, is to say there is an irreducible value to such high-order processes like intention forming, which gives meaning to free will. The sum of neural signals was greater than its parts.