Untrue.
CAP'd means Captains Advancement Program and it is a merit based program allowing the Commanding Officer of a seagoing command to advance a Sailor E-3 through E-5 to the next highest paygrade. There are a limited number of Sailors CAP's per year and its not done to increase watch rotation flexibility.
Most nukes are E-5 soon after reporting, so I'm pretty sure the "instructor" is BSing you or passing on some sort of sea-ditty.