Flatpak installs and shares runtimes. That's what makes it so stable, regardless of your distro.
So yes, if you install 1 KDE app from Flatpak, you will have the KDE runtime. But that is true if you install 1 KDE app while on Busybox as well. It's the subsequent KDE apps that will reuse the dependencies.
Which is often not the case. For those of us with slow internet connections, flatpack take hours to download programs that would otherwise take seconds.
That's the entire draw of Flatpak - I can have applications with out of sync libraries and they just work. That's a big big headache with system provided packages.
So yes, if you install 1 KDE app from Flatpak, you will have the KDE runtime. But that is true if you install 1 KDE app while on Busybox as well. It's the subsequent KDE apps that will reuse the dependencies.