He does, but then what? There aren't any visible opportunities to know the Bird Trio from Kanto, or the Ho-oh he's seen so many times, and so on. However, he has the chance to know Reshiram.You do realize that by this logic Ash has a reason to revisit every region, right? There are quite a few legendaries that he hasn't seen yet outside the movies.
About the episode, simply awesome. The bound Meowth's scenes was as cute as it was funny, and the plot entertained me. Can't wait for more!