It's not entirely true. Copland microkernel was in (hidden) in System 8.5 onward, and provided preemptive multitasking, file mapping, timers etc. It's just that the only real 'task' on it was the old cooperative OS.
So not entirely true -- it's "only" true from the standpoint of typical User Experience.
So not entirely true -- it's "only" true from the standpoint of typical User Experience.