When Kraken was first released, there was discussion over how its spell worked. The two big questions were (1) whether devour or damage came first, and (2) whether devour would occur if the damage killed the last troop.
The wording “Then” was a direct response by the developers to question number (1). I don’t recall a direct answer to (2), but I have to imagine it was intended to work like Emperor Khorvash, who didn’t stun additional troops if the damage killed the first two before the stun could be applied.