Does a motorized bounty hunter or any biker not natively having mimitism lose it when they dismount? The paragraph sounds like they have to possess the motorcycle skill (which dismounted bikers do not have) to have mimitism, but how is that different then not being impetuous on the bike and regaining impetuous off the bike?
When trooper lose Impetuous it is applied to the trooper and both trooper profiles are the same trooper, on the other hand the Mimetism is kind of an "on-going effect" applying to troopers with a skill and when you dismount you no longer have that skill. As currently written, which is kind of consistent with how those things work on non-seasonal troopers as well.
The lore reasoning for why they have mimetism is to do with the bikes kicking up snow as well so inclined to rule no mim-3 when dismounted.