Are you saying that only (human?) biological brains can be GI, and that whatever intelligence is, it would emerge from a pure physics-based simulation?
Both of those seem questionable, multiplying them together seems highly unlikely.
I think the argument is simpler than that. I have a PC, if I wanted to emulate an old Nintendo system well enough to play I dont have to emulate from the physics upwards.
Even though every NES in existence is a physical system, you don't physical level simulation to create and have a playable NES system via emulation.
1) intelligence and consciousness seem to be linked and we don’t understand consciousness. It may be physical but not purely chemical and classic physics.
2) Even if it is purely chemistry and classic physics, the development process may matter. Whole brain simulation may get you a simulated lump of flesh with no electrical activity.
Not currently explained != not based in physics. Modern physics does not have an explanation for how gravity works at the quantum scale, yet anyone who'd seriously argue that quantum gravity is not physical would be laughed out of the building.
Both of those seem questionable, multiplying them together seems highly unlikely.