The valence flat bands in transition metal dichalcogenide (TMD) heterobilayers are shown to exhibit strong intralayer spin-orbit coupling. I show that symmetry constrains the spin-dependent complex phase of hopping terms in an effective tight-binding model of the valence flat bands. A perpendicular electric field causes interlayer hybridization, such that the effective model becomes equivalent to the Kane-Mele model of topological insulators. The proposed model can be used as a starting point to understand interactions and the experimentally observed topological transitions.