Not a bug. Read the description again. "Pyke strikes in an X-shaped area, blinking to champions and executing those below a certain flat amount of health. Enemies in the X that are not executed take damage equal to this amount.When a champion dies in the X, the last ally to assist also gains full kill gold, and Pyke can instantly use Death from Below again for a short period of time.
This works both ways for Pyke so basically even if you don't get the kill you'll still get paid for it as long as it's in the X. This can also be abused for when you don't even participate in the fight you can get some ez money, but it'll still be difficult to time and you might end up wasting it for later.