That list of keys maintained by the archive administrators are signed by debian developers. That is how the archive admins can be sure that the key is in some sense legit. Otherwise where would be the root of trust?
The requirement for having individual keys signed by Debian Developers just makes it easier for the archive administrators to decipher which keys they want to add to their root of trust. The upload system does not check those signatures at all, they do not need to exist in the slightest as far as the upload system is concerned.