Stumbled into this before while browsing the store.
Click the link, hit buy on the free version, lo and behold, you also own the $23 full version.
No idea how long this will last, unless the game is doing a surprise announce that it's going F2P overnight and they botched the launch, I'd say it's safe to say this is just a good old pricing error.
Can confirm it's the full game, I downloaded it, loaded it up, and popped a quick easy trophy, something you can't do on trial versions of games. Get in while you can!
Fixed link to free version — editor
There's no free version listed. And adding the game as-is from the above link requires the full purchase.