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I have zero motivation to spend any time at all finding this out myself, but let me guess, this somehow involves a blockchain.

It actually doesn't (except for the fact that PKI registration is handled on Ethereum).

From the Urbit homepage: "Urbit ID is a decentralized digital identity system. Your Urbit ID is a username, network address, and crypto wallet."

So yes it absolutely involves a blockchain.

Don't mean to be pedantic but those requirements do not require a blockchain.