Oakape:
Is it a case of lack of sim fidelity or is elevator effectiveness reduced at high altitude by approximately the same amount as the available thrust is reduced?
Two factors.
Firstly, this was a stalling demonstration in the sim, so the aircraft was very slow. We might have been back at .62 mach or so, or about 180 kts IAS at 39,000 ft. Remember that with underslung twins, the thrust can overpower the elevator at low altitudes. Yes, there is less thrust at altitude, but that thrust is still opposing the required elevator motion.
Secondly, it is not simply the lack of elevator authority, but the stick forces required (not Airbus). The force required to push the stick forward was surprisingly high and counter intuitive. Normally at altitude the stick is very sensitive, so the need to push with both hands was not usual for that altitude, and could be confusing.
And that is why we do these demonstrations in the sim.