It would make sense for it to be roughly based around 2513; during the uprising of the Eridanus Rebels, and the commencing of UNSC's retaliation with Operation: TREBUCHET. I imagined that the New Mombasa map was more for the sake of homage, or just a project for fun~ Perhaps ground to promote a future Covenant addition. However, I always saw it as being very difficult simply modeling Covenant creatures, and getting them to properly function in the ArmA III engine.
By no means is this an official answer. I am not a dev for this mod; just wanted to get my foot into this thread, which I have been wanting to do for some time, ha.