I would be inclined to ASSUME (a very dangerous assumption by itself!!!)
that the Rally Bonus is made to be given to units which can take advantage of some sort of shelter.
I always ASSUMED the hill TM bonus not like a Terrain Advantage but a reprhesentation of the difficulty a downhill attacker can have shooting not at his level.
If you imagine to be on a slope with someone shooting from below, I don't believe you are able to hide somewhere. Is more like the shooter is engaged in a more difficult shot.
Just my rule interpretation.....
(Crossposted on BGG)