In a way this is comparing apples and oranges.
Slate winners are determined by USNA...only those in Admissions know if someone is a slate winner. A slate winner, unless there are some unusual circumstances, will receive an appointment -- since this individual will be "charged" to the MOC for appointment record keeping.
A principal nominee is designated by a MOC on their slate. There is no guarantee that a principal nominee, fully qualified or not, will receive an appointment. Though, the traditional practice is that fully qualified principal nominees, more often than not, will receive appointments.
I am guessing you maybe meant to ask the question in reverse? That is...does being designated a principal nominee mean a candidate is the slate winner? The answer is not necessarily. It depends on a few factors, but the biggest hurdle is being fully qualified...a candidate NOT fully qualified, even if designated a principal nominee, will not receive an appointment.