The Filamon had gone through great lengths to stop the Dark Zephyr from taking advantage of the situation and crush the escaping cockpit that held the body of Venerable Osaia Dukes.
As the escape vessel of a mid-tier expert pilot, its speed and defenses were anything but average!
Though at least part of its structure had suffered a fair amount of damage from Tusa's transphasic grenade, it still retained more than enough functionality to make it back to friendly territory.
In fact, Venerable Tusa never intended to kill Venerable Dukes in person. It was enough to destroy the Quadknife and remove an expert mech off the board. The outcome of the battle wouldn't fundamentally change if the expert pilot actually made it back.
After all, what would Venerable Dukes do once he returned? It was extremely unlikely that the Sundered Phalanx just happened to have a compatible expert mech on hand.