So, why did she slap him?
I think the most likely is that she heard or saw the annoying friends coming, and was embarrassed. The other options seem a bit silly assuming she really meant to kiss him at the slumber party. If it was an accident, then saying she didn't want to be kissed without a confession makes sense.
But now we're in a bad place. Itsuki basically reemphasized her no, I don't want to be with you. (in case leaving the country wasn't enough) but it'll probably screw with his head a bit.
And he things Iori slapped him because she didn't want to be kissed.
If he had just been able to use words... or just say yes to her question... grrrr.