According to the abilities file I have just read, Mountain Troops is a collection of ability (i.e. one symbol to includes more than one abilities) that includes:
Mountain Fighter : 25% combat bonus and 1 extra protection to this unit, only in Mountain, Wooded Hills and Alpine terrain.
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Mountaineer: Provides a 25% move bonus to the whole stack in Hills, Wooded_Hills, Mountain or Alpine terrains.
So those troops are indeed giving a movement bonus, but I guess the Mountaineer ability should be changed so that it works only at the unit and not the group level. My 2cents
