the information appended with a commit (username, time of commit, commit message)

it's not that, it's the cryptographic signature of the commit's contents with a private key, which allows verifiers to attest the integrity of the commit and authorship through the corresponding public key. The problem is that anyone can write anyone else's name and address in the author field. The signature would mitigate this impersonation problem.

And ultimately, that's a good thing. The article just puts into question the overall usefulness of it and how GitHub in particular handles this process.