Cynthia's Garchomp wasn't the only Garchomp shown flying, there was a Garchomp in the Summer School Arc in Sinnoh who did the same thing.
But a Dunsparce flying like that? It should not be possible. In an episode during the Johto series when Dunsparce were shown flying they were struggling to stay airborne because of those tiny wings. This Dunsparce however was shown flying as well as a bird with fully developed wings.
Maybe they changed that because a certain Dunsparce is gonna follow a certain boat, yknow wut im sayin.
nvm I'm just salty he didn't catch it...