My money is on fake.
Why would they make a cold beverage the promotion in the middle of winter? Starbucks is smarter than this.
Also it just says new pokemon, not generation 2. It's very possible it will just be a single new pokemon (or a handful). This could be anything from a rotation of the region-specific pokemon (e.g. the USA could have Mr. Mime spawning at Starbucks) to a release of gen 1 legendaries rather than gen 2 (e.g. whichever Starbucks sells the most of the new beverage will have a Mew spawn).
Obviously I'm just speculating here. I think it would be odd that they'd release generation 2 before any release of generation 1 legendaries. I also think if they made a promotion this time of year it would be on a hot beverage.