It isn't 100% clear and to be honest I would be fine with it either way.
The summoning ability says "before he attempts to cast a spell" (translated by me, I have the German GHB, so might not be the exact working in English) so it is clear that he doesn't cast the spell to create summoning points.
So you could argue that the ability is preventing him from casting the spell (which he doesn't) but not from spending the "spell slot" for summoning.
You definitely need one "spell slot" to try and change the constellation and cannot use that one for summoning.
So I guess it comes down to whether you want to take the fluff text into account (which IMO hints at the Slann not being able to do anything in the hero phase) or not.
Fun fact: when we discussed how summoning could work before the GHB2018 was released, someone here (could have been me, I don't remember) suggested that we could create points by using a special spell that is cast but does nothing else.
That way it would have been clearer since it would obey the normal magic rules.