GreenAcid is right. The Bladevanes may target units in cover, whose cover saves apply as normal.
Additionally the Jetbike rules say you may ignore all terrain, unless you sart or end your move inside it.
So as a conclusion you may hit the models inside a pice of terrain, while ignoring said terrain for all other purposes, except cover saves.
I could swear there was a rule back in 3rd for exactly this type of situation, were a skimmer, jetbike or jump infantry that wanted to interact with a model in terrain (eg. Tank Shock or chain snares) could not ignore terrain for this round, but it seems to be gone.
The FAQ even says it ignores this Terrain.
- BRB FAQ ver. 1.4 wrote:
Q: If a skimmer tank shocks or rams an enemy unit that
is in terrain must it take a Dangerous Terrain test? (p71)
A: Only if it begins or ends its move in terrain.
I must rule with eye and claw — as the hawk among lesser birds.