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Red and Green are former champions right? So considering they were both in the PWT's champion battles, it could be considered that, since all of the champions are canonically defeated, and since the other protagonists aside from Red aren't present in the tournament it can be assumed all of them are former champions. This is furthered by Alder taking part since he's no longer the champion of Unova.
Taking into consideration that after each battle in the PWT your Pokemon are healed, while when fighting the champion you do so after fighting theElite Four without healing your Pokemon. The PWT Champions tournament could be considered equal to any other protagonist fighting the champion without facing the E4 before hand, which is a much less impressive feat.
TL;DR The PWT ain't shit but a bitch and doesn't contribute much to Nate/Rosa's strength
