Looks like you're right - I could have sworn there was an interrupt in the RTS example.
Anyway, in this case I think conditional aborts will be your better bet. Place the stun animation branch to the left on your idle/attack branch under the selector and add a Lower Priority conditional abort to the Stun Sequence selector. When Has Received Event evaluates to true it'll interrupt any lower priority branches (in this case, the idle/attack).