You are describing attributes of the NeXT-based OS X but calling it MacOS 9.
MacOS 9 was the final version of Classic. No preemption, weak virtual memory, weak cooperative multitasking, no virtual machines for anything.
MacOS 9 is the OS that ran inside the Blue Box on Mac OS X for backwards compatibility.
You're mistaken.
> MacOS 9 [had] no preemption
Classic Mac OS has had vertical retrace (VBL) interrupts since System 1.0 (which are a form of preemption). But suppose we disregard that.
>> MacOS 9 had a preemptively scheduled kernel.
The GP is correct. Applications run cooperatively within the Blue task, but the Blue task itself runs preemptively with other MP tasks (which an application can create).
Very very little used it. About the only classic app I ever heard of that could even use pre-emption and background execution were some demanding Photoshop filters. Enough that Daystar Digital built multiprocessor Mac clones just for that alone:
https://lowendmac.com/1995/daystar-genesis-mp/
Within the MacOS kernel it is as coop as ever, and there were no different OSes inside virtual machines. No, Tenon MachTen does not count.
I think the clincher is this line:
« MacOS 9 ‘solved’ that by running the classic OS in a single process. »
That is absolutely unambiguously referring to Mac OS X. OS X ran what was even called Classic in a single process: the Blue Box.
MacOS 9 did not do that. It does not have pre-emptive "processes" as such at all and it does not run any older OSes in anything.
It is clear to me that this is not talking about the nanokernel, but if it were talking about the nanokernel it would still be wrong, because that did not appear in MacOS 9. It first appeared in MacOS 7.1.2 which was the first PowerPC release, I think, but that version had no tasks or anything.
But the first version with tasks and asymmetric MP support was in MacOS 8.6.