The covering-map conjecture for nonsurjective developing maps

Let $M$ be a compact conformally flat Lorentzian manifold with developing map $\operatorname{dev}:\widetilde M\to \mathbb{R}^{2n+2}$, and suppose that $\operatorname{dev}$ is not surjective. The covering-map conjecture. The developing map $\operatorname{dev}$ is a covering map onto its image. This would strengthen the structural consequences available from the discreteness of the holonomy group; the source does not provide evidence that the assertion has been resolved.