Wraithseer doesn't have that keyword, sadly, and an FAQ has not fixed it. I think the Eldar FW book has had some keyword problems (no CHARACTER keyword on the PL for Shadow Spectres, and no JUMP PACK keyword on him or on the Shadow Spectres for about a year IIRC) so I think this is just an oversight. Similarly to how the Wraithseer doesn't have a statline that is compliant with the Codex Wraithlord statline - instead, the stateline is based off the Index Wraithlord statline which is worse. So, until Forge World updates it, the rules in the book don't allow this.