When they said "compatible", they didn't mean the disc could be compatible with DVD/CD players. They meant the HVD player can be compatible with CDs and DVDs, since they got the reading proccess to be similar at some points. There's a red laser that reconstructs the holographic image, and it's possible to make it capable of reading CDs and DVDs.
That would not be possible if they used cubes-shaped media, with lasers comning from two different directions. Such thing would never read a normal disc without having a whole CD/DVD drive attached to it.