PackageKit isn't a package manager in the same sense as what I meant. It's more like a one level above "front end" to be able to manage different package managers with the same program. This means that "Software Stores" that use packagekit like Gnome Software or KDE Discover will work on most Linux system with whatever package manager is used in the backend. For example on a Fedora Workstation, packagekit makes it possible to install, update and manage both rpm installed through dnf, Flatpaks and if I wanted, Snaps, while on a Debian based system it would be able to manage your apt stuff, or on Arch packages installed through pacman for example. But from what I heard this also makes it a somewhat clunky and slow piece of software that has become kind of clunky and hard to maintain over the years, so its also an interesting question whether Cosmic is going to use it.