Yes, that's what I mean. For example suppose you had this mixed solution (core comes from debian repository, "kubuntu personality" from kubuntu repository).
Then debian maintainers release updates for their packages - which they tested and validated in systems that use only other debian packages.
Next time you update your system, it may happen that the new version of debian components are no longer compatible with the kubuntu components.
Debian won't wait for or check if every distro who uses their "core" has tested debian changes and released compatible new packages of their own.
Probably most debian based distros simply repackage many base debian components with minimum or no changes, but they know those releases are compatible with their own "customized" packages, and can have control of their dependencies.
Edit: I didn't address one of your questions directly: No, developers and maintainers of a linux system component (as kde, and even the kernel) not necessarily are the maintainers of a specific distro packages.
For example, kernel decelopers and maintainers release a new kernel release independent of any distro. It's up to the distro maintainers to test and package this, then make it available in their repo.