ouvouvn This simply means that for some packages (the stateless ones), Solus provides its default configuration somewhere under /usr/share/default/...
but the user can override them system-wide by placing his own configuration files under /etc/...
(where ususually find the config files on many distributions).
This way it's very easy for a user to revert to the Solus default : simply delete the file(s) that have been placed under /etc.
And for the packager it's good too: there is zero risk overwriting changes a user could have done locally on his config files since the default configuration files should never been touched by the user.
The thing is that there are still many packages that aren't stateless.