The kernel image itself is neither packaged, nor part of the rootfs (nor upgradeable in place) - and it has neither met either of those criterias in the past.
The sole purpose of the kernel package you're referring to, is providing the necessary dependency meta-data for additional kmod packages - tying the (externally-)installed kernel into the dependency framework of the package manager/ opkg. By itself, it's an empty -useless- metapackage without any content, but nevertheless necessary - and yes, this works as designed.
Slipped my mind that its included in sysupgrade and that I need to extract the image and dump it manually as sysupgrade does not work the way the node is setup