I think it's ok.
If you draw a line from the object ball to the mirrored pocket, it will intersect the first cushion (left end cushion) at 1 diamond plus x, (1+x). The extension of this line back to the right end cushion will be at 2(1+x), or 2 diamonds plus 2x. He's saying to take that 2x and divide it by 2 to get the correction, x. Seems perfectly reasonable.
Of course, this all assumes a mirror reflection rebound and that the mirror is located at the cushion. Because of the finite contact time with the first cushion, and the fact that the ball scoots over a little bit while in contact with it, the geometrically exact location of the mirror is not easy to place. But this is splitting hairs.
Jim