To be fair, their PPC-on-Intel emulation that let old versions run of apps (and the entirety of OS9, IIRC) was a technical marvel on a number of levels - especially considering the hardware limitations of the early chips.
Mac OS 9 never ran on Intel. That was a separate migration path, where Mac OS X would run a Mac OS 9 VM, but this "Classic" experience never made it past PPC.
On PPC you could either dual-boot or run OS9 (and 8?) apps seamlessly on an OSX desktop, which was pretty impressive.
The seamless emulation of OSX-PPC apps on an Intel processor was extremely impressive though. I remember the majority of stuff working surprisingly well with little slowdown (though this might now be rose-tinted).
Mac OS 9 could run apps written for anything from System 7 onwards, and sometimes even System 6 apps though that was hit or miss. It could even run 68k binaries.
Mac OS 9 inside of “Classic” (the VM that ran OS 9 inside of OS X) wasn’t especially seamless, but what was seamless was “Carbon”, a transitional API that allowed developers to build apps that ran natively on both Mac OS 9 and Mac OS X. It didn’t take nearly as long to port code from OS 9 to Carbon as it would have taken to port to Cocoa, so many early OS X apps were Carbon ports.
