3:58 PM, Monday October 19th 2020
Oh, my mistake - I had seen it but by the time I finished the critique I forgot about it, so thank you for the reminder.
Generally speaking I treat the treasure chest challenge as being an optional "extra final boss" after Lesson 7. It is optional, and once you complete lesson 7, you'll have earned the completionist rank. If you wanted to do it earlier though, I'd say that you should just have completed lesson 6 (with you have done).