The PPC -> Intel transition also happened, and yes devs had to do work to make it happen, but it doesn't mean it is unheard of or impossible.
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 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.