If Ash isn't facing Takuto here wouldn't the first episode in which we saw DRai be called "Darkrai appears" though? I think this is Ash vs. Takuto.
Darkrai appeared in the first round of the Sinnoh league. Which was dedicated mostly to Ash vs. Nando, which wouldn't make sense to re-name [though if they did it probably would've been like "Sinnoh League First Round! The Legendary Pokemon Appears!" or something to that effect]. If the episode is Takuto vs. Random Trainer, I'm guessing what they mean by "Darkrai appears" is that we get to see it in an actual battle or something, because we've never seen it battle.