He did say his studio software runs under OS9. G5s don't run OS9, nor do Intel Macs. I use OS9 for most music making on a G4, too. Classic doesn't cut it for a lot of software. Unless you build your music from loop libraries and samples, current generation software doesn't have anything over what was around 10 or 15 years ago. Some of us are slow and need a decade to master the subtleties of our instrument.
If you're doing live recording with lotsa tracks, then you want a new Mac. If you're doing arranging or composing, or MIDI stuff, you don't need one.