Forging the Blackblade triggering when it should not bug

Forging the Blackblade triggered on my opponent's turn (to be clear, it was their Forging the Blackblade) when they had no creatures down, killing all of my creatures.

The text contains the word 'Then' between it's two effects, making it clear that the second effect is contingent on the first effect triggering. This is clearly a bug, please fix, thank you.

https://www.youtube.com/watch?v=K68jTl7UMFg