One just need someone who makes it credible that the hash and timestamp were not tampered with.

This is the problem blockchain solves. That need for someone trustworthy disappears.

Even posting the hash on Reddit would do it for most people.

What? People can edit their own posts. Admins can edit other people’s posts. Anyone who thinks reddit is immutable is an idiot.

Reddit isn’t going to commit fraud for some random person.

Spez has already edited posts. Pay people enough and they will happily edit a database.

And that random person is probably not able to hack the database undetected.

Already happened. But also the user can “hack” their own entry.

What’s also missing is chronology. Blockchain fixes not only the document contents but also puts a minimum on the age. Also if the hash is signed then authorship can be attributed.

Recomputing lots of hashes isn’t difficult.

Yes it is. That’s whole reason blockchain works.

A blockchain doesn’t add any trustworthiness on its own.

The contents of a document can’t be trusted, but anything a blockchain (internally) verifies can be.