Most airline sales aren’t really sales, but this is legit. It’s a bit of an odd marketing play, though, since you don’t have to buy a Starbucks drink to get ‘buy one get one’. You don’t get a gift card for a drink when you buy one, get one. There’s no Starbucks tie-in whatsoever except, it would appear, some marketing dollars from Starbucks to cobrand the promotion.
This strikes me as a genuinely good deal if you’re traveling with someone in and out of Alaska Airlines cities.