Mac OS 9 came out in 1999, and had an official funeral in 2002. The Mac Mini came out in 2004, and the official lowest supported OS is Mac OS X 10.3 Panther which came out in 2003. From my research, there were multiple efforts over the years to back port an OS released 5 years prior to the release of the target hardware. Finally someone did it, and after testing it out it really just works.