this would allow you to use the virtualbox GUI and configuration utilities, but run the VM under KVM virtualization, correct?
Yes but from the looks of it, this isn't using the libvirt backend, so you won't be able to use/configure your Box VMs with virt-manager/Cockpit, in case you were planning on doing that - aka it's not going to turn VBox into yet another fronted for libvirt. But it does indeed use KVM.
Is there much advantage to this over using virt-manager, or cockpit to configure your VM's?
Putting aside the easier-to-use GUI, it has certain unique features not found in other opensource hypervisors, such as seamless mode. Also, it may have better compatibility or performance with certain operating systems - for instance, when I wanted to test GhostBSD on virt-manager, I ran into lot of issues, such as not being able to install it in UEFI mode, mouse not working etc; whereas it ran and installed without any issues at all using VirtualBox.