No, Apple committing to the project DID in large part get the G5 developed. Without a commitment of a large Apple order IBM would never have had any reason to add things like AltiVec support.
By the way, I'm no Apple fan. I have an 800mhz G4 that I only bought to 'go OS X' and found that the GUI performance in Jaguar is so sluggish it feels like using a 7000 series sometimes. My built-for-next-to-nothing Duron 1.1ghz with Windows 98 leaves in in the dust for GUI, although the G4 beats it for just about any big process in Photoshop. I really with I'd stuck with my beige G3 - the G4 is probably my only near-pointless computer purchase.
As the G4 runs speedily enough under Linux and OS 9, it can only be down to the huge overhead of Quartz. I'm sure the sluggishness of Quartz still has an effect on much faster G4s/G5s.