EbonJaeger The guest additions iso has definitely never been included by us;
I realize that now. And I remember downloading versions of it from virtualbox.org in the past, after which VirtualBox was the default application to open it. That's apparently how it got installed in the same folder as VirtualBox, itself.
That didn't happen this time. I'm sorry for any stress I might have created in my attempts to get to the bottom of this. I suspect it may not have been installed on Solus together with VirtualBox is because the guest additions probably contains non-FOSS drivers and such. Thus, on systems that strive to be FOSS-only, VirtualBox can be installed without the additions.
Why the web site no longer includes a link to download the guest additions is a mystery I don't want to pursue. My Windows 10 VM is now a pleasure to use, and I thank everyone who was involved in making it so.