The Flight Simulator team at Microsoft takes their work pretty seriously. I very much doubt this is prerendered; they know the cost of BSing their (very hardcore) fanbase.
When you say "Flight Simulator team", are you talking about the team that ported the subLOGIC simulator to the IBM PC?
Seems unlikely - subLOGIC themselves did that port, and Bruce Artwick continued working on Flight Simulator for Microsoft, and there was significant work on Flight Simulator, particularly with regard to realism, in the decades after the initial release of the Microsoft version.