This is actually correct. Tested it myself last night.
Sigh. Here's another test for you, one to prove you're wrong...
Have everyone dps from max melee range, and the person you want to be the defuser stand underneath the boss. When it dies, the person underneath the boss will get it every single time.
Why do you think in Methodicals video, all 4 times in 16 man the sage healer got the defuse kit even though he never attacked the droids once?