> Another alternative is these messages are broadcast through a public and decentralized ledger.

Trouble is, the "decentralized ledger" can only reliably timestamp if there is a financial incentive that motivates peers to burn hashes, stake and vouch for blocks etc. Without that financial incentive the whole blockchain breaks down to something inferior to even the PKI distributed systems described by Lamport in the 70s and 80s.

That's the major fallacy of "blockchains" as a technology, they are always a disguise for token speculation, they require native minted tokens of speculative value to operate.