If Cap still had his TimeGPS (which he likely was sure to hold onto), that links to the prime timeline... think of it as a tether. So it's likely that was used in some way in getting oldCap back to his original timeline. Whether he used that to jump back to Prime 2023 ala how he and Tony went from 2012 New York City to 1970 New Jersey (no platform needed during that jump, that could explain why he didn't end up on the platform after he 'missed his window'), or else he enlisted the help of the alternate timeline's great minds (whether Howard, or Hank Pym, or even the alternate timeline's Tony Stark) to use the timeline that the GPS device was linked to (the prime timeline) as a guide point in sending him back.