What he means is, that every magister turns hostile as soon as you escaped the fort.
Some quests are undoable after this point, if you did not do them before, like Yarrow & her dad or the source dogs.
This is not true. I sneak out of the fort pretty much every time I play through the game. Then I come back and do the little quests I missed. Yarrow is the only magister that is not hostile when you return.
