The source discusses the movement of a writer's fingers under two different conditions: while writing and while not writing.
“Zayd moves his fingers while writing” and “Zayd does not move his fingers when he is not writing” may both be true; the conditions differ.
A proper comparison is:
“When Zayd is writing, his fingers move.”
“When Zayd is writing, his fingers do not move.”
When subject, time and other qualifications also coincide, affirmation and negation concern the very same conditional claim.