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Cool Idea, can one use hashes like sha256 to prove that the article original content has not been modified? Is that already in IPFS?

It's already built-in. URLs in IPFS are Base32 encoded SHA-256 hashes https://docs.ipfs.io/guides/concepts/hashes/

That doesn't tell you anything about the original source, though, just the version that got put on IPFS.

which surely is the best possible without the original author signing it?

All that can be assured is that the content that the person mirroring it and linking you to it is as they intended.