So if no data was leaked from the tax agency or from the users, then the leaked "digital signing documents" must have belonged to the only remaining party, which is CGI, so perhaps they were just some marketing documents about the benefits of their digital signing service?

The original phrasing from the attacker, from the website that put the data up for download/sale, was ”documents (for electronic signing)” which implies that they’re documents that would be signed in said system. I would take all of this with a large helping of salt though. CGI claims it’s not real production data anyway; maybe it is and maybe it’s not.

The best case scenario is in line with what CGI claims: these are lorem ipsum fake docs from an old git repo for a test instance of the system.