Problem as others pointed out is that no matter how you design the system, even with Google a completely as trusted party in this, proof-of-authorship, timestamping-service, it can
only work if every original content creator (or platform, like SO) gets on this train.
Say you're a content creator that maybe doesn't want to use this service, simply hasn't heard about it yet, or accidentally publishes a whole archive of original articles they forgot to sign with the proof-of-authorship/timestamping API just before they get scraped by an ill-intentioned content farmer, that quickly uses the API to sign/stamp the articles to themselves before the content creator can. They don't even need to publish them right away, they can drip them out over years and have the proof-of-authorship signature to "prove" their authorship.
If we would actually trust this system, it means that the real original content creator is shit out of luck. There's no way they can prove their authorship in a way that distinguishes them from a scraper, that part remains the same with or without this system--but what would be new is that the scraping non-author now has some sort of extra claim of authenticity over the actual original content creator.
I see no way around this. Unless you devise a way so that all content written (anywhere, any time, on any medium) is immediately signed with a proof-of-authorship, such a system will be giving power to bad actors to claim authorship of any content that happens to slip through without being signed.