Grey found a place for Cormac to hide. He did not need to take anyone with him if there's a chance he would confront those from the Twilight Faction. Alone, he was confident he could escape from an encirclement, but once he has to look out for someone else, the situation becomes difficult for him, just like the case of when he tried to rescue Arkhos.
Cormac knew he could not go with Grey, so he had no issues with hiding and waiting for him. Even if he could, he would not want to cross the Twilight Faction. A few Forces with multiple Peak Sovereigns can act as they want, but a Force like the Vallarian Tribe that only had three Sixth stage Sovereigns, it was basically a death wish.
Leaving Cormac behind in a self made cave, he rushed to the place where the core was located. As long as the members of the Twilight Faction were not in the area where the core is located, he would not bother with them.
