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But that does have to be enforced via incentives/cryptography, and with reference to a clock.

Well it is enforced by the protocol on which all node agree and there needs to be a mechanism for syncing clocks across nodes (also defined by the protocol) but they exist, e.g. https://ieeexplore.ieee.org/document/8946264

Not 100% sure what ETH is using here but there is a mechanism.

If someone would want to fake timestamps, they would need to deviate from the protocol and would thus not be on chain.

You are simply raising the fundamental difficulty with blockchains. How do you know what is the true chain in an adversary environment (the internet)? You gave us the question and no answer

Validation of the longest chain, agreed upon by the protocol which the nodes are running. This is the absolute fundamental of this technology. This is not new and it has been working for more than a decade.

* which the majority of nodes are running

That's not how the technology networks. That naive implementation would leave you wide open for sybil attack

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