I had been fighting The Messenger, Ironclads and the wooden druids and their spells always went off (as you said, these are not proper spells). To check that it worked against kith I went and attacked Arkemyr and it does interrupt his spells.
I have mainly been using fists/mace to get my stacks up which is reasonably easy to do on most mobs. Understanding that the spell disruption only affects kith is good to know but also quite annoying (in Baldur's gate 2 it was all mobs). As this class is quite gimped already and now to restrict it to the kith isn't great.
Awwwwww. The appeal to this class has really dipped.
Thanks for the reply @Boeroer. Your insight and testing of this is most appreciated.