Although philosophers have explained variously the correlation between simplicity and truth, they generally agree that simplicity signals truth. Unless utilitarians can otherwise justify it, searching for a simple moral theory means searching for a true theory.

This is the fallacy of the undistributed middle. You say, essentially,

• All searches for truth are searches for simplicity
• Utilitarianism is a search for simplicity
• Therefore utilitarianism is a search for truth

While I understand that utilitarianism being a search for simplicity is evidence that it's a search for truth, that does not give you license to automatically assume the worst of a theory you dislike.