But ACL2 is a logic. We can prove theorems about the model.

Theorem. MC 'mult is a multiplier
(implies (and (natp x)
(natp y))
(equal (lookup 'z (mc (s 'mult x y) (mclk x)))
(* x y))).
This theorem says that a certain program running on the mc machine will correctly multiply any two natural numbers.
It is a statement about an infinite number of test cases!
We know it is true about the model because we proved it.