That wasn’t working, but just reinstalling the KDE notification packages made them default again.
sudo pacman -S knotifications knotifyconfig
//EDIT: and it doesn’t survive multiple reboots, it gets back to defaulting to this ugly notifications. not sure how to force it to stay on KDE’s default notification.
//EDIT2: seems like proper fix https://unix.stackexchange.com/questions/603186/wrong-notification-system-in-kde-plasma
mkdir -p ~/.local/share/dbus-1/services/
ln -s /usr/share/dbus-1/services/org.kde.plasma.Notifications.service \
~/.local/share/dbus-1/services/org.kde.plasma.Notifications.service
Also, the KDE case needs to be placed before the Xtrem case, or else if you have Xterm installed on your KDE installation, this is what is picked first instead of Konsole.