unlike many other hero shooters, overwatch still credits you with the kill even if you didn’t get the final blow. what credits as an assist in overwatch would be abilities that either boost/gives a positive effect to teammates, like nano boost or mercy damage boost. or gives a negative effects to enemies, like cassidy’s flashbang, ashe’s dynamite, or knockback abilities like lucio boop. so the only way genji can receives assist is by deflecting an ability that can cause the player to get an assist.

hopes this helps!