Because I did things a little differently as I described in a question in Lecture 110. I had no problems with the enemy shooting projectiles and destroying each other - when I saw that the ships disappeared in the Lecture I literally said โWhat the hell is going on!!!โ. Then I remembered, oh right, so, doing it my way saved myself the trouble of having to solve that problem.
Ahhh, I like it when a plan unwittingly comes together .