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Kabu appears the same in the anime as in the games, and Ash has met other Galar gym leaders before. This means either Kabu was a kid younger than Ash in Journeys (unlikely), he's absurdly old (since he appears older than Friede, who is claimed to be Ash's son (also unlikely)), Horizons doesn't take place in the same timeline as Ash's (likely), or that Horizons is completely unrelated to Ash's journey, and thus Friede is unrelated too (highly likely).