To me, many of them are not what I would call impossible. Looking them over I think I could recreate many of them in a 3D program easily enough. The only ones that fit that name seem to be the ones with a horizontal element connecting two vertical elements. Even then many of those where the horizontal is on the bottom or top, like T, U, V, W, seem to work. I wouldn't call this a collection of "impossible" letters but a nice typeface inspired by an impossible object, just like it says next to the name.