My guess is you're correct.I fed my fish every day for a few months. Cloves gone forever. Nothing impacted in the reef - corals or fish. It works.
Compare this to a one or two time treatment where tanks had issues.
5 days is not likely enough. This is an over-time treatment.
Edit - it would assume that fembendazole is released unchanged in feces. Curious - why not just treat the tank?