This is a pretty tricky area. The NEC does not require exit signs. They are required by OSHA, the Life Safety Code and building codes for most occupancy. If your building did not legally require an emergency system, and it seems it didn’t, only a reliable source of power is required. That is not necessarily generator or battery backed. The installation is probably OK. If the building were legally required to have an emergency system the exit signs would have to be wired per NEC article 700. This would require a second source of power.