AnandTech just wrote that A6's CPU core is neither A9 nor A15
It's a custom design, somewhere in between A9 and A15. Kind of like what Qualcomm did with Snapdragons (Scorpion and Krait).
I'd say this is a smart move for Apple. Given the volume of devices that they ship, they can easily benefit from building custom chips that incorporate all features that they want and leave out those that they don't. But more importantly, this means that they have better control of the release cycle of their chips and final products. And in the worst case, they can always fall back to stanadrd ARM designed core.
There had been reports saying that no company can ship A15 in large volume before the end of this year, and this clearly put a huge constraint on Apple if they went with A15. I guess this time their own design does pay off.