It sounds like you agree 900 MHz will be much better than 2.4 GHz in this non line of sight application so long as the obstructions are trees and foliage and not terrain.

I would expect to be able to send a few bytes between two stations maybe 400m apart through non-line of sight forest with 900 MHz and both stations at the same elevation. Certainly the grandparent quoting miles would be for mountain top to mountain top.