All this early-pay credit counts Active-Duty pay-periods AFTER you become a Traditional Reservist or Guardsman. You still need 20 good-years.

If you are an AGR or TAR (Navy guys), you obviously get your retirement pay the day after you retire.

They are talking about days when you are on AD or MPA orders as a Reservist. Your UTA and AFTP / GTP periods do not count for the earlier pay-date treatment, but obviously still count for retirement points.