He did say that in Hoenn but I am not sure he said he was only using his Sinnoh pokemon anywhere. With Hoenn it was a little different because if you recall he was inspired by seeing Ho-Oh fly off towards Hoenn (or at least the general direction) and he wanted it to be like his first journey through Kanto.
With Sinnoh he wanted to explore the world more and find new pokemon and battles being inspired by Gary. I don't think he said anything about only using Sinnoh pokemon and it really shows just how tough the Sinnoh League must be if he is calling on his other pokemon. The number of participants seems a lot smaller than previous leagues which might indicate there is some tough competition. And then there is the Elite 4 and Champion which hopefully we'll see Ash go up against them right after the league if he wins.