Still guessing, but I remember thinking at this point also that I had made an earlier mistake.
I think it only appears like you don't have the flaps. I doubt you've made a mistake before this point, the earlier unsinks are pretty obvious, and there isn't really anything *too* tricky before this step.
But this step ... this step is rough. I failed hard with my homemade tissue foil because with the first builds, you are really going to need paper with good crease memory for the unsinks. Tant would be great except for it's way too thick at the end for the legs (which is probably why he switched out in the video).
I thought about the step a little more and hopefully can add some clarity. When you do that first unsink of three, make sure (again) that only one layer of paper is being unsunk - the remainder of the paper gets pushed back inside the flap (there is no gap in the center of the top at this point, if you have a gap - this is definitely where you and I erred). The second unsink is pulling that tab outside the flap - now there should be a gap. The third unsink is the remaining paper that evens out the right side.
