It's simple: the login shell is bash and fish is set via Konsole options. This keeps compatibility with applications that expect bash (eg. Flatpak), while being able to enjoy fish.
I like the look of the new gnome console 'kgx' so I'm trying to see if there's a way to do something similar but there doesn't appear to be. The old 'gnome-terminal' doesn't have rounded corners. Plus if you use the Tiling Assistant extension to add gaps, then 'gnome-terminal' adds wonky extra space around the the bottom and I think the right side too.