The DIGIC accelerator takes the heavy lifting from the main processor. It is what deals with processing the crazy amount of data that come from speed and resolution. The DIGIC processor is left to deal with rendering and whatnot.
It also deals with autofocus, the "learning" and the cross-type R1 stuff as well as exposure. Features like shooting video and stills at the same time, and offloading them to separate memory cards.
Think of the DIGIC Accelerator as a powerful GPU taking some of the processing duties from the CPU.
Dual DIGIC processors in some cameras used to be a thing. The difference here is the accelerator has specific duties, it's not just two processors working in parallel.
You're just guessing though. Even the tech guys from Canon don't know what exactly does it do. My opinion is similar to yours – it helps the Digic X. And as you said, there used to be dual Digic in some cameras. This is probably something similar but the camera doesn't need double the speed so the accelerator is just a little brother.
My point was that the Digic accelerator probably isn't designed for a specific task like "AI autofocus" so an absence of it may not mean losing features. It may be that 30-ish MP is still fine for the Digic X alone, but for 45Mpix it needs a help. Or for cross-type (as you mentioned).
Or, the accelerator helps with the "basketball autofocus" feature and the R6iii won't have it.
One more point is that Digic X is not the same processor in every camera so the one in the R6iii may be more powerfull.
Anyways, just saying that having no Digic accelerator doesn't mean much.