They can't get a 3+ cover, unless nightfighting is in effect. They get their coversave from the Jink special rule which is on page 38 of the BRB. The rule about moving in the Assault phase is on page 45 of the BRB, in the bottom right-hand corner box. You are right though, the rules about doing this Assault-phase move after shooting are a bit unclear. I would say, though that the Eldar Jetbike Specific rules in the box are additional to the standard bike/jetIbike rules described on the rest of page 45, so the things in the box can be done as well as all the other things. I hope that makes sense to you?