Can't subscribe to a folder of a user whom is not in GAL
It's not possible to subscribe to a calendar of an account that isn't in the GAL. It appears that Evolution checks that the calendar account exists in the GAL before subscribing to it and so fails.
This was discussed on the Evolution list in the thread starting at https://mail.gnome.org/archives/evolution-list/2020-May/msg00136.html
I was able to see the calendar by manipulating the underlying .source files, so there was no permission problems, I just couldn't subscribe through the interface.