If you believe that what we describe as "consciousness" is emergent from the ideas a material brain develops about itself, then it's in fact not logically possible to have a world that is physically identical to ours yet does not contain consciousness. So indeed, premise 2. sneaks in the conclusion.
To illustrate this point, here's an argument with the same structure that would similarly "prove" that gravity doesn't cause things to fall down:
1. In our world, there is gravity and things fall down.
2. There is a logically possible world where there is gravity yet things do not fall down.
3. Therefore, things falling down is a further fact about our world, over and above gravity.
4. So, gravity causing things to fall down is false.
To illustrate this point, here's an argument with the same structure that would similarly "prove" that gravity doesn't cause things to fall down:
1. In our world, there is gravity and things fall down.
2. There is a logically possible world where there is gravity yet things do not fall down.
3. Therefore, things falling down is a further fact about our world, over and above gravity.
4. So, gravity causing things to fall down is false.