That was actually a cart I created specifically to demonstrate how camera() works. So I just made part of my _draw() function manually draw a little screen preview in the corner based on where box was. It looks cool, but it took some work to create. I hope that answers your question. (Let me know if you have more.)
Oh, that's great to hear! :) I'd be happy to make those demo carts available as well. You can load them in PICO-8's command line by using the following commands:
