I can't see any mention of 'legal' criteria in the mainstream reporting of this. The map that went viral just says 'Most popular'. It colours Cuba and N Korea in red as if Coke really is the most popular drink in those countries.
Sure, I'd agree that Irn Bru's preeminence is clearly more impressive given the market conditions, but that's not the claim Irn Bru's retweet or any of the recent viral craze has been making, so it's worth clarifying. If you were Cuban I'm sure you'd appreciate somebody pointing out that you're not coloured red.
It's all a bit of silly season rehashing of social media activity anyway - Irn Bru's unusual market dominance in Scotland has been known and talked about for years now.