The (-4 . "&")(70 . 1) filter will be true if bit 0 (value 1) is set (bitwise logical and), thus (in this case) will be true for 70 group values of 1 or 129 (1 + 128), which is what we want: retrieve closed lwpolylines with no regard for the plinegen status.
The (70 . 1) filter by its lonesome is absolute: be true only if the 70 group value equals 1, so it will only retrieve closed lwpolylines that do not have plinegen set.
Does that clear it up?
:)