didn't Barry said he wanted to battle Volkner?
maybe he and Ash will get there at the same time and they will both argue who will go first against him.
and then Flint will come along and say: "hey, this sounds like a good opportunity to cheer up Volkner. why won't we all battle at once! me&Volkner vs. you&Barry!"
Barry did say it? I don't know. In DP163 they will meet him, so maybe you have a point, they are going to the same place. or maybe Barry is actually returning from Sunyshore. however, I think that Barry won't appear is Sunyshore, he appeared enough.