I think the meaning behind this is that the largest amount of work doesn't go into the distro itself and mostly into the KDE software. The Ubuntu base is not developed or maintained by the KDE team, they basically just "borrow" it as a platform for their KDE suite.

I daily drive KDE Neon.

Sometimes install scripts don't work as expected, since things check if you're on Ubuntu or Mint or whatever specifically and "Neon" doesn't match their regex. It's usually not a big deal and fairly trivial to solve.

Regardless, I've actually started to get away from the command line and have embraced the app store. Discover is actually pretty darn good and has lots of the things I want to install. I can choose if I want to install from Discover via Apt, Flatpak, or Snap.

I usually install Flatpak stuff. The Steam Deck has taught me that Flatpak is generally as good or better than actually installing via apt - you don't need to wait on your distro to update sources, and you aren't adding random PPAs. Sometimes you need to fudge the permissions with Flatseal, but it's a one-and-done thing.

I use Microsoft Edge as my browser (yes, really - the Chromium version is just as good as Chrome, it has nifty vertical tabs, I get news on my "new tab" page, and all my settings are saved there). I use Thunderbird for mail, plus Steam, Zoom, Discord, etc. Surprisingly few KDE apps are preinstalled, to be honest - the only KDE apps installed are the ones I want anyway.

I think they refer to other desktop environments. I've never had any issues installing other software on my system, it works just like any other Linux distro.