That’s not how you calculate potential energy. If you were to put both portals on either side of a moving platform, you would pass right through it, because they are both moving at the same relative velocity. But if you make one of them stationary, you work like trowing a ball forward in a moving vehicle.
In this example, if you calculate as if both portals are stationary, the people are moving towards the portal, the portal isn’t moving, and thus they fly out the other side.
That’s not how you calculate potential energy. If you were to put both portals on either side of a moving platform, you would pass right through it, because they are both moving at the same relative velocity. But if you make one of them stationary, you work like trowing a ball forward in a moving vehicle.
In this example, if you calculate as if both portals are stationary, the people are moving towards the portal, the portal isn’t moving, and thus they fly out the other side.