HDCD was (is) just an improvement of standard Redbook CDs, and they play in regular CD players, but the player does need HDCD decoding circuitry to take advantage of it. It's not a competitor of either SACD or DVD-Audio.
If Bill Gates bought the HDCD technology, he must have some use for it. At the time it was developed by Pacific Microsonics, HDCD was an advanced dithering process that made redbook CDs sound better-- but perhaps it has other applications as well?. Cheers. Craig