He used one of the three Backstage themes in the Entropy video, he used static from the game on 5:41 of this new video on Buckshot Roulette's dev, he was shown the game on stream and clearly liked what he saw before snapping back and going "ew ferry" for the bit, clearly he played the game if he used Backstage1/2/3, he should make a video on it just saying.
#Pyro make video on Echo by Echoproject?
vapid cradle
slow flare
This doesn't convince me he's played Echo. Just about all of Echo's sound effects and music is under common licenses. I've been jump scared by Echo music in the most random places.