If it was no movement cost to attack, but 4 moves total instead of the 5 my skirmishers have then it's a nerf. That's functionally 5 moves still, but 1 of them is always dog-eared for a specific mission instead of being able to use that extra move for another move or pillage action. I think we would have to remove logistics as a pickable promotion for skirmishers though, because logistics with no movement cost to attack on a ranged unit is very good.
I would be okay with that. I'm thinking in the context of Mongolia where I gave them +1 attack, which makes that UA even stronger. It would also slightly weaken skirmishers in the hands of Denmark, Inca, Songhai, and Iroquois, who have terrain movement bonuses that augment skirmisher movement a lot. Those civs can't use those 5 moves to such powerful effect if one of them is reserved only for an attack.