Strangely enough, from their desktop FAQ:
https://en.opensuse.org/openSUSE:Desktop_FAQ> openSUSE installer provides three officially supported desktop options. There is no default choice
Coming at this from another angle, their installer isn't super opinionated. The KDE radio button is at the top of the list on the desktop selection page.
Does that make it the default choice? I guess it does by some definition.