I have a theory;
Shimano know their market well. From what I can see, Dura Ace is bought by two kinds of people; firstly, it is bought by racers and shredders who demand excellence, and actually use high end gear right to the limit. It is also bought by monied MAMILs (which I would imagine is actually the main market share). This second group, while wanting the latest top end gear, is never going to use it anywhere near the limit, and is also rather conservative, so making a 20.8mm rim with the Dura Ace name on it for these people is perfect. It is guaranteed to fit in whatever frame they put it in which means more people will be able to use Dura Ace stuff, so Shimano will sell more, and get more exposure / advertising across the board. It also adheres to the conservative notion of 'stick to what you know', so the older guys who focus on nostalgia in cycling also get that desire met.
This doesn't explain why they sticked with the C40 name and published false rim data. Also sneaking new data into an old spec document is wild.
Ok. here is my theory: they decided to sunset their rim brake clincher line in their next generation (9200) and simply it's not worth to develop a new rim brake clincher rim at this point. They know that there will be only a handful of rim brake frames available for the public in 4 years, so no reason to put money into a new alloy / carbon rim.
The new tubeless compatible, disc specific rim has the new height and width. The tubular rims got updated too, so the pros will be covered, whatever happens with the disc brake UCI approval in the future.
I find it extremely troubling that none of the cycling websites covered this story. I wonder how they call themselves journalists.