I still don’t understand what you see as the problem.
On google maps if I measure the distance between two locations 90 degrees of longitude apart at the equator, it’s approximately 10,900km. If I do similar for two locations at about 50 degrees latitude it’s approx 5800km.
This shows google maps is considering the great circle distance, correct? The line on the map is clearly curved.
In the sim, if I select a route from London to somewhere in western Mongolia (90-ish degrees of longitue at 50 degrees north) the distance is about 3100nm, or 5800km.
Which surely demonstrates distances in the sim are great circle. What am I missing?