After you posted your email with them, I contacted the devs myself. They replied last night saying that they were indeed going to post the update, but as I was quite busy last night I forgot to mention it here.
Great. Obviously you are more persuasive than I am. I just hope that they will confirm the Humble update as well for me.
Here's the reply:
sorry for the late response. In the past it was really difficult to give updates to GOG (and other platforms) other than Steam. Nowadays it is way more easy than ever. However, in our case the bug was to not display credits, which also was set as a minor setback since nothing game-relevant was broken or missed. This is also the only fix in the steam version.
But good news: we finally found a way for the GOG version and a updated game should be available in the next days.
Thanks for understanding,
I don't know about more persuasive, it was probably a cumulative effort. Don't forget that I also wrote to GOG about this and supposedly someone from the product team was reaching out to the devs.
I'm sure that now they've prepared a DRM-free build of the latest update to the game that the devs won't have any trouble uploading to Humble too (although you're probably right to prod them).