I don't remember very well because I made this video 2 years ago. However, as you, I remember that I also had problems. I think that you have to remove all the screws on the back of the computer. If you see the video at 01:34
, there are 6 screws in the back, 4 at the corners and 2 more in the middle (1 for the little cover and other one behind this cover). Did you remove all this screws? I believe that this last screw is the one that is blocking your cover. Isn't it?