I don't need to be told a thousand times that I'm a -blam!-ing idiot, I fully understand that you're supposed to kill the knights.
What I don't understand is why it ends the strike before they even spawn, if they're the target, but I guess I'm the only person to ever -blam!-ing exist who thinks that that's a problem.
I'm not calling you an idiot, I made the same mistake when I first ran the Strike myself. I'm just trying to let you know that killing the Knights won't count if Savathun's Song is killed first.
I hope you manage to finish the quest on your next attempt.