yes because that is what it has to be in order to prove the theorems, in order for the theorems to be true. Because if for example you take in norton theorem the "equivalent" resistance in series with the "equivalent" current source and the load resistor, then you just can't prove the theorem. Which means that the supposed "equivalent" resistance and current source (as defined by the theorem) are no longer equivalent i.e the current and the voltage in the load resistor will not be the same with the "equivalent" components in series as they would be if you had the actual circuit.