The key word here is back-up. They could have used a genuine #9 because Turpin hasn't sown the position up himself but there were genuine ways they could have circumvent the system and actually made use of their affiliates. Promoting Levi into the NRL squad just seems brain dead and takes up a spot that could have gone to a quality free agent.
I don't think the signing has anything to do with Turner. Brisbane simply didn't want him enough and were only willing to take him to get something out of the Warriors. They thought he was on a development deal so it would have been low risk, as it stood he would have been just another running half taking up space when the options are well and truly covered.