Words like “employee,” “payee,” and “detainee” support the generalization, crudely stated, that words with the ending -ee designate the person affected in the specified way by an action performed by someone else.
Evaluate
At first the author is considering a blanket rule that goes like, .
Then she considers absentee and realizes there's no someone else who is absenteeing the absentee.
(Whereas there was a someone else who is employing an employee, someone else who is paying the payee, someone else who is detaining the detainee).
So the author adds a little caveat, another qualification to the rule, so that absentee would no longer qualify: .
Let's look for .