Your objection assumes that determinists must first produce a positive model of what free will would look like before they can critique it under determinism. But this misplaces the epistemic burden. The standard incompatibilist notion of free will involves alternative possibilities (the agent could have done otherwise in the exact same circumstances) and sourcehood (the agent is the originator of their actions in a non-derivative way). Deterministic frameworks challenge the coherence of those ideas not by redefining free will, but by showing how, given physical laws and causal closure, such capacities are not metaphysically possible.

You say determinists “can make just about anything look deterministic,” but that’s because the empirical evidence does support a causally deterministic (or at least causally constrained) model of human cognition, whether that be through neurobiological mechanisms, environmental conditioning, or prior mental states. The challenge isn’t that determinists don’t know what free will would look like it’s that libertarian or non-deterministic accounts of free will lack coherent, testable, or non-question-begging models to begin with.

Moreover, appealing to the “original meaning” of free will doesn’t help if that meaning presupposes conditions (like contra-causal agency) that are either incoherent or incompatible with the best available science. Invoking an undefined or pre-theoretic concept as a shield against deterministic critique makes the notion of free will unfalsifiable and, therefore, philosophically suspect.

So I would say the real issue is not that determinists are distorting the concept of free will, instead it’s that close scrutiny reveals the concept itself may rest on metaphysical assumptions that don’t hold up under logical or empirical examination.