a prorate fee will be billed to reactivate

To my reading, both ‘fee’ and ‘billed’ indicate the customer will be charged an amount to regain access. If it were as you suggest, I think it would have been worded differently, to provide more benefit to the customer. Maybe something along the lines of:

‘When you are ready to come back, your membership reactivation will be prorated to reflect freeze term payments.’

As it is, I can see both interpretations. Which might be on purpose to surprise people who turn up at the gym after a few months break, banking on an eye roll and the fee to be paid so they can get on with their workout.