But they are distributing binaries of the game to the public for free[1] -- the text you quoted describes the exact opposite situation to what is happening. In this particular case you cannot charge separately for source code under the GPLv3 -- see section 6(d).
Of course, if they are the sole copyright holder, they can dual-license things under a GPL and proprietary license (which is effectively what they are doing here -- the DogWalk binaries available from the linked page are not GPLv3 binaries because they are not following the GPLv3 requirements). But this situation is absolutely not permitted under the GPLv3. Otherwise a company could fork a GPL'd project and just avoid releasing GPL'd source code by charging $1B for the source code.
Of course, if they are the sole copyright holder, they can dual-license things under a GPL and proprietary license (which is effectively what they are doing here -- the DogWalk binaries available from the linked page are not GPLv3 binaries because they are not following the GPLv3 requirements). But this situation is absolutely not permitted under the GPLv3. Otherwise a company could fork a GPL'd project and just avoid releasing GPL'd source code by charging $1B for the source code.
[1]: https://studio.blender.org/projects/dogwalk/gallery/?asset=8...