I thought I'd remembered reading in a Playstation forum back when I learned of this CD's existence that t was included as a bonus CD in a Playstation Underground (or similar) magazine. However, it looks like from this site:
http://vgmdb.net/album/1400 and a few other similar sites that discuss the CD like this
http://lostvgm.com/vgmrush//album.php?id=358 that it was a sold in a limited edition version of the game as it appears you've discovered the same thing
When the tracks from the first Spyro game emerged from Stewart's Vault, I think the tracks that appeared had notes written by Stewart where he discussed the names he gave the tracks. I believe he said they were all nouns that he wished to describe what he was feeling at the time he composed them, similar to the notes he did for the tracks that appeared in the Anthology. In fact, Rain was one of the tracks that was in the Spyro 1 vault. One of the other tracks from the Spyro 1 edition of Stewart's vault was Orbit, which was the title track of the first Spyro game. Stewart said in the notes that it was for William Orbit If you are fan of William Orbit like I am, you will notice that Stewart gives a tip of the hat to Orbit with his use of the acoustic-like guitar sounds which are similar to what William Orbit sometimes uses in his recordings. In fact, the Orbit track in the Spyro games reminds me a lot of the track El Nino from Orbit's fourth Strange Cargo album Hinterlands, in which it sounds like Orbit is giving a tip of the hat to Stewart with it's use of the synthesized bass you hear in the Rhythmatist album and a lot in the score of the Equalizer's first season.