Open is not the same as patent-free, the two things can coexist (and they do in the case of webp).

It’s open to write the code, but in order to be authorized to use it you have to get a permit from Google. You can’t eg.: fork from Firefox and use their permit (as you implicitly could with patent-free). Plus, Google can rescind their patent grant at any point, which they are bound to do once they secure ownership of the internet.