Well this it all true I am not sure this applies to 123. 123 stores all its coordinates in WGS84. I would think that 123 will do the area calcs in 84 in the back. I am doing this now with distance. I take a passed in lat/long from Collector (which uses a WGS84 service) then use radian math to clac the distance from the point given to the current location.
123 does not really have a map - that is just for viewing your location on it. I do not think 123 would be asking the map, or any service, the area math since you will probably be offline.
All this is my guess. So hopefully 123 can give us some more insight.
Thanks