Comment by jerf

The real problem is you need a real-number-valued universe for this to work, where the measurer needs access to the full real values [1]. In our universe, which has a Planck size and Planck time and related limits, the statement is simply untrue. Even if you knew every last detail about a piece of fairy cake, whatever "every last detail" may actually be, and even if the universe is for some reason deterministic, you still could not derive the entire rest of the universe from it correctly. Some sort of perfect intelligence with access to massive amounts of computation may be able to derive a great deal more than you realize, especially about the environment in the vicinity of the cake, but it couldn't derive the entire universe.

[1]: Arguments are ongoing about whether the universe has "real" numbers (in the mathematical sense) or not. However it is undeniable the Planck constants still provide a practical barrier to any hypothetical real valued numbers in the universe that make them in practice inaccessible.