If timed right, which is usually by chance of the window, I can be unblockable. In my XBL Ermac days I used to always try ending the combo into relaunchers on robots and Sheeva. (aaHP x2, jump forward, jp popup)
Yeah maybe in the video, you're right about that. The closer to the ground they are, the higher chance of being blockable. That's my take, and it goes double for Nightwolf.
I bet it's very technical, having to do with the actual differences in frames it takes for a character to fall from a popup combo and anti-air punches.