One reviewer of the new iPP https://www.macstories.net/stories/ipad-pro-2021-review/ and his comments on the MKB compatibility.They did a poor job explaining it and it will be interesting to see what the iFixIt X-Ray scan shows of the two. They created a LOT of confusion around .5mm though (including magnet placement shifts in their accessory design guidelines)!
I'm just glad I won't need to be trying to find a way to sell my old MKB to buy a new one. As I had gambled that it wouldn't change when I returned my 2020 IPP during the return window to buy the 2021.