Illustrative Mathematics 8th Grade missing lessons?

I was just looking at the 8th Grade Illustrative Mathematics course and noticed that unit 1 includes lessons numbered 3, 5, 6, and 7. I also noticed that unit 1 lesson 3 begins with an exercise with no prior instruction. Is this because Khan Academy chose to make the course available before it was finished? Or am I missing something? I'd noticed a few "missing" lessons in the 6th and 7th grade Illustrative Math courses, but generally not so many gaps as this one starts with. Also, I hadn't noticed missing instruction prior to exercises in the previous courses.