How exactly does Tezos instantiate any blockchain?

Tezos seems very good at self-amending itself.
Now, how do you go from the self-amending capability to the universal blockchain instantiation?

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The position tezos paper claims this:

"In this respect, a protocol can be described by only two functions:

  • apply which takes a Context and a block and returns either a valid Context or an invalid result (should the block be invalid)
  • score which takes a Context and returns a score allowing us to compare
    various leafs of the blockchain to determine the canonical one. In Bitcoin,
    we would simply record the total difficulty or the chain inside the Context
    and return this value.
    Strikingly, these two functions alone can implement any blockchain based
    crypto-ledger. "

Any proof that apply and score can implement indeed any blockchain?
And what are they exactly?