Trouba is one of my least favourite players in the league, but this is a really tough play to analyze in real time. There are so many awkward split second decisions happening here I think it would be a mistake to be sure of anything.
For all we know Trouba was leaning in to make a big hit, noticed the dude was going down, and was trying not to knee him in the head so jump as high as he could. The elbow is a thing sure, but he could've just been leaning into the hit too and when it was clear he wasn't making contact he further extends to cushion the impact.
Honestly I don't know. My guess is with you all that this probably would've been a dirty hit if he connected, but there's way too much going on here to really be sure either way.