ولقسم الاستعارة التي لا يمكن اجتماع طرفيها في شيء عنادية
A metaphor whose two terms cannot coexist in one respect is called opposing.
To call an existing person “nonexistent,” intending that the person is of no practical help, borrows the idea of nonexistence for someone who actually exists. Literal existence and literal nonexistence in the same respect are incompatible; the opposition creates the emphatic figure.
The sentence need not be treated as a confused denial of the person's external existence. With an adequate contextual indication, it communicates the absence of benefit, not a metaphysical claim that the person has ceased to be.