[Gameplay] Devourer Stacking Issues

It appears that there's an issue with the Devourer enchantment. My friends have insisted to me that you have to actually kill each monster yourself to receive the stack for it. I wasn't certain of this, as the item's descriptions says it stacks off of takedowns, but then I saw a video of Kayle jungle using Devourer. Kayle's teammate killed the dragon, but although Kayle assisted with it, she didn't get the 5 stacks for it. The moment I'm talking about is at the 13:50 minute mark.

https://www.youtube.com/watch?v=5TEb6z33FYk

Steps to Replicate:

1 - Have someone build a smite item and grab the Devourer enchantment. 2 - Fight any jungle monster with a teammate, but dragon is most noticeable. 3 - Have the champion with Devourer back off from attacking and let the teammate finish off the monster.