You are confused by the definition of "cured". The curing process is when die off occurs after rock is removed from the ocean. What is left over, is the live rock.
You are asking about base rock, which is live rock that has very little to no growth or life. You could use 50% base rock and 50% live rock and still have a successful system. And yes, the base rock will become live with time, so long as it has been seeded with actual live rock in the aquarium.
This is an oversimplification, but answers the question.