I have to admit I'm confused by this. Apart from the extra megapixels, it doesn't sound like a big change.
It also seems strange to be updating the "baby brother" without a similar bump to the R5 - weren't they were originally released together?
It doesn't seem that "strange" to me. The R5 is still a solid camera in both the R lineup and against it's competition. Despite some FW bungling of late, Canon has continued to update it to keep it relevant. It can easily go another year, year and a half, before getting an update -- which I think it will. I doubt we'll see the R5 II until well after the R1 ships because the R5 II will likely borrow a lot from the R1. As an R5 user and probably not an R1 buyer, I'm happy w/ not seeing an R5 until 2024, assuming this is the case.
The R6, OTOH, became little lost in both the R lineup and the competition as a lower resolution body when the R7 was released. A lot of R6's initial buyers were enthusiasts that wanted more than an R but just didn't want to, or could not plunk down $3700 for the R5. Of course, there were also pros that were perfectly happy with lower resolution because it aided in processing and keeping storage costs reasonable.
But with the R7's intro, enthusiasts had the new choice of a MP $1500 body. Sure, crop sensor, but in many genres that's even desirable or a non-factor. And even for the pros who initially bought into the R6, 20mp these days for a mid-level body is low. (Not to mention that Nikon is rumored to be reading the Z6 III, presumably with AF borrowed from the Z9). So the R6 II will address that issue to rejuvenate it and also spread some distance between the R7 and R6 choices.