I think checksums can be used to verify that particular messages are the same, but ultimately I think there just aren't that many protocols for ensuring that every provider has every message. Some messages could get dropped and the solution is that providers will just check again - there's no protocol level guard against it.

Absolute integrity is a lot less important in systems which assume that inexpensive manual overrides will be possible - because you save a ton by simply deciding to build a system that 'only' works 99.9% of the time and intervening in 0.1% of cases. Obviously blockchains don't work 100% of the time but that's the premise for their increased costs.

Newsgroups were run by a secret cabal for a long time. Beyond that the system just doesn't preserve integrity; a given provider can censor some messages and that's generally something you live with or switch to a better provider.