You're not wrong and ty for making this post I've been meaning to make it myself but forgot too. No you do not need to get a headshot with throwing knife for the perk to activate, most of the time's i get headshots with throwing knife and it does not activate incendiary blade. While i was doing trials i experienced this a lot, where people should have died but didn't because the burn effect didn't activate especially when I was dead. This was extremely annoying and I hope this post gets a lot of attention so it can get fixed. I also find it funny how bungie felt the need to nerf throwing knife and couldn't even bother to fix this glitch.