For the purposes of the article, git isn't just being used as a database, it's being used as a protocol to replicate the database to the client to allow for offline operation and then keep those distributed copies in sync. And even for that purpose you can do better than git if you know what you're doing, but knowledge of databases alone isn't going to help you (let alone make your engineering more economical than relying on free git hosting).
For the purposes of the article, git isn't just being used as a database, it's being used as a protocol to replicate the database to the client to allow for offline operation and then keep those distributed copies in sync. And even for that purpose you can do better than git if you know what you're doing, but knowledge of databases alone isn't going to help you (let alone make your engineering more economical than relying on free git hosting).