Incorrect instructions for kubuntu?

The instructions here:

Flathub - Apps for Linux

Kubuntu Flathub Setup | Flathub

Set up Flathub to install and update apps on your system.

do not work, at least for me. I have installed Flatpak. I assume that is what “back end” refers to. but the command

sudo apt install kde-config-flatpak

gives the error message

Unable to locate package kde-config-flatpak

Something is missing here but I don’t know what. I have installed flatpaks before so I know the system works.