Hmm I thought the rules were meant to apply during non-combat movement as well. I think, as seems to happen with many rules in this game, we’re reading the same thing differently as it’s a bit ambigous.
1.8 Mountains (Optional Rule):
Units are subject to Mountain rules on the first round of combat when crossing a mountain border and on all combat rounds in a mountain zone.
Combat: All Attacking land units have -1 Attack.
Movement: All land units (except cavalry) have their movement reduced to 1 when subject to Mountain rules. Units cannot blitz across an Enemy Mountain border or into Mountain terrain.
To me, the section on Movement is separate from that of Combat. Meaning, the Movement section applies to all forms of movement, be that combat or non-combat. But I think I see how you are reading the initial paragraph as the rule being specific to combat movement. But that also to me is just letting you know for how long the Mountain rules apply if attacking a Mountain border or full Mountain territory, not necessarily that that is the Mountain terrain rule, but that that is how you would apply the combat for the Mountain terrain rule.
Could obviously be wrong, but I’ve always played that terrain rules apply in combat and non-combat movements.