I have 3 theories
What if they didn't beat the Elite 4? What is because they get a free pass to see you because they are related to someone important? No one would say anything in this case.
Theory 2 (the more likely one)
What if they have another team that isn't in the coding or just can't be found in the coding? They might just beat the Elite 4 and then switch out their teams.
Theory 3 (the obvious one)
How about this? They already beat the champion with a team that's not in or cannot be found in the coding and the one they use against you is what they are currently just training? This one seems pretty likely as they most likely won't get a free pass. They also can't just switch out teams in the middle of the Elite 4.
EDIT: They might have spammed healing items and used EXTREME strategy to beat them all as well.
Hope I helped!
-It's a secret!