Whether free will is compatible with determinism depends on what you mean by free will and determinism. If compatibilists and incompatibilists used the same definitions for these then they would both have to come to the same conclusions about whether they are compatible or not.

The possibilities are (1) It follows logically from the definitions of free will and determinism that they are compatible. (2) It follows logically from the definitions of free will and determinism that they are incompatible. (3) It doesn't follow logically from the definitions of free will and determinism either that they are compatible or incompatible.

The claim that both use the same definitions but come to different conclusions is incoherent.