Actually I just followed your link and checked the BRB FAQ, and it is indeed still in there. The FAQ has 13 pages and it is on page 10 in pink.
There are two sheets per page, there are numbers on the bottom corners on the one numbered 10 it talks about leadership and it is there.
Looks to me like you use it for both, as it doesn't specify either way. As to the OP, low dice phases where your slann is left twiddling his thumbs, or falls down a howl on 3 dice, is one of the reasons I've got tired of him and resorted to skink priests instead. Who cares if the hey miscast, so long as they IF Comet , Cuse or Harmonic Convergence?
The slann in my opinion is like most wizards a risk vs reward deal. I don't like the new slann as a party build I personally like to run him as cheap as I can so when the winds blow badly or you cascade you are not losing a quarter of your army. I have really tried to get focus of mystery to work but you hàve to start investing too many points in skink priests to get off the short range spells to swap them out. I have not tried wandering deliberation as I have been running a life slann but I could really see the appeal. At the end of the day the slann is unlikely to win you a game single clawed, but by adding his weight where it is needed magically he can be a real difference maker. So I feel you have to look at your list and your play style and ask what do I need to support or what button can I push to make things go from ok to brilliant.