I realize that the point of this is to generate buzz, but what's the point of buzz if you're going to follow it up with, "Ha ha, just kidding. We're not actually going to sell you the thing we're advertising."
The thing they're advertising is the shake, and they'll absolutely sell it to you. Now, you may not be interested in buying it if you don't get to try the funky straw, but advertisers know that a lot of people who see their ads won't be interested in buying the product. But some will.
And... would you ever have heard of this shake if it weren't for the straw? I wouldn't have. And neither, I'm sure, would many people who actually might be interested in a chocolate mint shake.
This is pretty much the definition of a successful advertising campaign, at least in nerd circles. We're voluntarily discussing a novelty shake from McDonalds! I can't comment on how it will play to the wider audience, but it worked on you. And me.