Apple could have moved the 8th generation CPU-equipped models to 9th generation, but the 27" BTOs were already on 9th generation. And Intel did not release the 10th Generation until late May.
The 23" I could very well see on 9th Generation because it is supposed to be "cheaper" (and it would still be an upgrade from the current 8th Generation in the iMacs).
As for ARM-powered Macs, I am convinced that will not launch until 2021 and when it does it will be a MacBook Air.
They might launch late 2020 with the next MacOS update. Or thats the delayed switch to mini LED.