Basically true, though at the time in '98 that was generally taken for granted by consumers, the landscape for consumers was the binary realm of Macs and WinTel boxes. You could get Linux on the thing since it was Open Firmware based (I ran MkLinux on a Rev. B iMac and dual-booted), and YellowDog and other distros showed up not long after. Linux in '98 was still a very non-user friendly thing and was mostly for hobbyists and servers. As I recall, the movement to popularize Open Source software to the public only really started to pick up steam after “The Cathedral and the Bazaar” was published in '99 (with other OSS evangelism picking up steam as well).
I think you could run BeOS on the iMacs too, if you wanted less software and a different closed platform that was amazingly fast, and the BSDs were booting on it in '99 if you wanted a non-Linux open OS. But that was all very niche, if you bought an iMac it was likely because you wanted the newer, faster, cooler looking Mac.
