Yeah, it's an oddity for sure. I'm sticking to the "never attribute to malice what can be explained by goofups" explanation for now until I hear back from GOG about it though. :)
I've confirmed that the game neither shows up in the GOG catalogue through the website nor through Galaxy unless you click on the PC gamer link or elsewhere that directly links to the page. I've also confirmed that once added to your account, it is not available for download on the website, and that the GOG Downloader link leads to a 404 page. Regardless of the intention, that definitely is a website bug as nothing should ever point to a 404 page on purpose. :oP
I can also confirm that within Galaxy there are no downloadable links for this game and that it is only installable via "Install" within Galaxy.
I went ahead and filed a friendly support query to GOG support for this to let them know about the problems discovered and linked to this thread in my query as well. It's the weekend and this is a rather low priority issue so I'm not expecting a response until Monday at the earliest. I have a feeling that this is a new aspect to this in some way that there are just bugs in the website etc. that they need to feed some pizza to their devs to get it sorted out though. :oP
When they get back to me I'll post an update here to summarize what I hear from them.