I admit it odd the way they are handling this.
The link is to the Apple Store app in the AppStore. You have to install that app first and then open it.
Then click in the "Jump into the new year" link.
Then you can download the workout app for free.
They are giving out a code to make it free instead of just making the app free in the AppStore. This was the moves up in the rankings, but as a paid app. Others see it and then buy it because it is up in the list.
So if you are interested, download the Apple Store app and grab it before Feb 20 or so.