You read that right. It's apparently as easy as downloading the Battle.net app, using it to download World of Warcraft and launching the game. If you're on the fence about returning or have someone you want to show the game off to, now is possibly the best time imaginable, since you won't have to pay for the majority of the month. There's also a helpful link to Blizzard's beginner's guide, which can help you dive in and get started in WoW.
Probably a pretty good time for a promotion like this, with a lot of people having let their subs lapse during the long content drought, it's an easy way to try and ease some of them back into the game while also giving new players a painless way to explore whether or not WoW is the game for them.
Edit: Turns out it's not all new players. It's part of a specific email promotion.