Duke wasn't the first wizard to return from the Seventh Outpost; before him, four Level 1 Mages had already found the Fifth Outpost and returned to the Wizard Plane.
At that time, only a part of the wizards were at the Seventh Outpost; many others, like Duke, were on missions outside.
Those wizards on external missions ironically avoided the ordeal of facing attacks from Apostle level Awakened.
In fact, the casualties at the Seventh Outpost weren't severe; the biggest loss was Brandon's death as he sacrificed his chance to escape alone to buy time for others to teleport.
As for Hela and others, they must be lurking elsewhere; they surely have the capability to find the Fifth Outpost.
However, they haven't returned yet, indicating that they're probably investigating other matters.
Duke also doesn't believe Hela and the others would die; as long as they wish to flee, the indigenous of the Devourer Plane would find it hard to capture them.
