In order to get a cover save to use, you usually have to be in some kind of terrain. I think it'd be silly to include the line about being able to use the cover save against bladevanes if that never came up, and it'd only come up if your Reavers moved over the unit stuck in a piece of terrain, thereby moving over that terrain.
I've always played it where you can move regardless of terrain in the way because, as Thor said, you move over terrains with jetbikes, not through it.
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