You are right in saying that designer could act in any manner. But the million dollar question is do we know of any evolutionary mechanism that have the potential to mimick the action of designer i.e power to generate large amount of biological information required for the first cellular life to sustain? The “identical to- and indistinguishable” argument put forwarded by you becomes valid only when such a mechanism is discovered.
Note that my premises and conclusion of Intelligent Design (which I posted as a reply to Roy) were based on the origin of first cellular life. It can be falsified if someone is able to prove that functional biological information required for a cellular life to sustain could be generated solely through a particular evolutionary mechanism - Selection/Drift acting on random accidental mutations. In case I missed any other possible mechanisms, you can point it out.
As I described earlier, all the built-in cellular mechanisms would be absent in the hypothetical evolutionary pathway between first self-replicating system and first cellular life.