You are correct, it cannot take the trait.
Well that's just stupid...
I think many still see summoning in Starborne as a tactic on its own/main way of playing, instead of seeing it as a tool to assist your army. You can totally make a Starborne army with teeth/gas on its own, and summoning will just be a tool in your toolbox on the side.
Many lists will have a Slann/Kroak and maybe an Astrolith. Thats already 6 pts on average which is enough for 10 Skinks. Some lists will be very viable with a Kroak AND Slann in their list, which is an average of 8 pts.
There's several issues with the way summoning appears to work now.
First of it shouldn't be "just" a tool. It is one of our major faction/allegiance abilities, and arguably the "big" faction ability for starborne alongside the teleport. The "big" faction ability should not be a mere tool amongst many.
Second, it doesn't help that a minor support hero like a branchwraith can already put out more powerfull & reliable summoning than a slann + astrolith combined (on average). Making our mega-awesome faction ability rather dissapointing.
Third, there's obviously the fact that we go from flooding the board to barely summoning anything, which is just rather a large contrast.
Fourth, with everything being D3's it's just too unreliable and you will get games where you fail to summon more than a single unit of skinks if you're unlucky (And that's just bad rolls, I'm ignoring opponents who just murder your slann..) Alternativly, during lucky games you'l suddenly flood the board out of nowhere. Which is going to result in a very distorted view of how powerfull it is depending on luck, both amongst us as well as our opponents (that one guy who faced someone with 6 slanns that summoned 3 stegadons is going to be crying OP....)
As a minor sidenote, if you can actually use multiple casts per slann to generate CCP you end up on the other side where bringing 2-3 slann would be wildly overpowered, though the D3 does help a lot to keep that in check at least to some extend. But minor detail.
Anyway you look at it, on the whole this variant of summoning seems to be rather poorly thought out.