Slightly delayed response (didn't receive e-mail for some reason) ...
The cause of this was that I had moved an example sub-project (a self contained project within the examples folder) into another sub-folder within the examples folder. That second sub-folder also happened to be a workspace of the primary crate. Because of this, the originally moved example sub-crate was still being searched for a Cargo.toml which now happened to contain a path that was no longer valid. Because this all happened during the process of a bunch of local development, Cargo must have had the dependency for the invalid path cached on my machine and it never gave me any problems.
Had I tried to publish the base crate in this configuration it would have errored and I would have eventually found the problem. In this case however, Botev pulled directly from Github and ran into the error above.
After cloning the repo myself on another machine I got the same, head-scratching, error and really just by luck happened to track the problem down pretty quickly by disabling all workspaces (since it was something I knew I had added recently).
Anyway I just put this here in case anyone finds this post in the future. I've been meaning to file a bug report about the inadequate error message but I never could decide how to describe the problem and well... I'm lazy.