I don't know about you guys but, to me, the definitive argument regarding whether a new game means a new gen is being able to trade/battle. With the exception of RBY->GSC, once a new gen is released, you can never battle and can only do "one-way trading". Note that, after the GSC->RSE overhaul, they didn't need to stop tradeback. There really wasn't anything that stopped DP from having a Time Machine-esque feature to send Outrage Mence back to RSE, much less from keeping such a feature all the way to ORAS.
This was a design decision made by the devs. They don't want you to look back and keep on playing the older games with new toys, they want you to play only the new shiny toy they're trying to sell you. This means that you can only pick your best "bros" from the older game and send them to a new journey in the newer one.
And then we have this bit from the official site. It says we can send RBY mons to Sumo (I'm calling Sun/Moon that, sorry not sorry) and that's it. And we already know from another piece of info that XYORAS are excluded from that, so we know that the lack of a mention to them in the first sentence isn't an accident. Then we read the second, outlined sentence and guess what? You can send from XYORAS to Sumo via Bank!
...
"In the same way". No "between them", no "from and to", nothing. It's one-way, even if using the Bank. Just like RBY->Sumo, you can send XYORAS->Sumo.
It doesn't matter if Sumo is in Southern Kalos, if Floette and the Zygardes make an appearance in the plot, or if the game even uses the Kalos starters. It's a new gen.
EDIT-- No, I don't think that we would have a new gen even if we got 200 new pokémon in a new region and everything, if we still could trade. We already get new pokémon in every third game since Platinum, with ORAS being the worst offender with its new formes
which couldn't be used in battle against XY. Note how they simply restricted trading items and battling versus the "new pokémon" (new to TCPi, as to us they're "mere" formes). It doesn't seem they're doing it again this time.