I was trying to figure out the change as well - I've only used Tumbleweed through WSL before. Does it provide a desktop environment preinstalled or is it a 'bring-your-own' deal? (if not, that seems to be the big thing that Kalpa brings to the table?)
Tumbleweed comes with desktop environment options. You can select from a few.
I guess you get the atomic system, but with Tumbleweed you get snapshot backups anyway.
One of the main advantages of Tumbleweed is the extensive testing pipeline. I'm not sure how a derivative would be able to offer a similar experience