Good observation, but that's a little too old to be relevant, back then there were only three generations and they could "mess around" with themes like that. Now Pokemon is a well-oiled marketing machine with a TON of content -- if they're going out of their way to advertise something like Sinnoh, it means there's a specific reason for it. Not to mention how the 20th movie and Pokken also are also pushing DP themes, not just the TCG. We're getting DP hints across the board. Other than they're doing it "just cuz," the question is why...
Maybe they're trying to signal to us, diamond and pearl remakes are coming sooner than we think?
Almost like a "Objects in mirror are closer than they appear"?