If a random poor third world person can easily obtain a valid certificate, then any attacker can also do so - identity theft or totally fake identities are a thing despite all our best efforts and it's naive to assume that this will be magically solved in the future; and if it is not easy a random poor third world person to do so, then they won't, and all their real, valid user-generated content (e.g. real cell phone videos from conflict zones) won't be signed, and the society will consider unsigned videos as valid.
Furthermore, any attacker with the desire and resources to create a disinformation campaign can simply recruit a new real person to sign each deepfake campaign, just as criminals now hire money laundering mules (I mean, that expense would be less than the actual effort of creating the media) and any intelligence agency using deepfakes for propaganda can literally create new valid identities by fiat (just issue new real passports/birth certificates/whatever for nonexistent people) that are indistinguishable from real people as far as google or anyone else abroad can verify.
In essence, what you describe would work if and only if we had a global, trusted database of all people worldwide that doesn't allow for fake people. We're very far from that, and the obstacles for that aren't technological, it's definitely not something that Google can solve.