Astronomic definition of "winter 2014" does not equal a commercial definition of "winter 2014".
There's obvious overlap between 2014 and 2015 when taking "this winter" (as in the coming winter) as the release window, but given they stated "winter 2014" and not "coming this winter", I would conclude that Nintendo are aiming for a November or December release. If they'd gone for the latter release window, I'd agree that there's a higher probability of a 2015 launch.
The game could still slip to 2015, but right now, I think a 2014 launch is the plan. The original argument of "winter begins December 21st" is a highly selective, and forced, argument. Commercial shopping seasons and distribution periods do not hinge on the shortest day of the year, they hinge on the most profitable time of the year, which, in the Northern Hemisphere, are the winter months of November/December.
Like I said, a 2015 launch is possible, but I don't think that's signified by "winter 2014".