he didn't jump... he cast Misty Step which I believe is a bonus action.
Jump is a reaction that allows him to switch places with a Dupe when the Mirror Image roll turns against him (so basically, he always gets all three dupes to die first as long as he still has his reaction.
Quote
Then Eithne unleashes thunder that causes the horrible lieutenant of the Plague Lord to shudder before it casts Misty Step and slips northward.
End Quote
ug, forgot to link the spell