OCI defines a spec for container images. Both the Linux and Windows kernels support containers, but the way containers work is by running on the parent kernel

OCI defines a spec for container images. Both the Linux and Windows kernels support containers, but the way containers work is by running on the parent kernel (as opposed to traditional/micro VM's or unikernels). So if you want to run a Linux container on windows you need a VM running the Linux kernel to provide the host kernel for the container. The same would be true even on Linux if you needed to use a different kernel for the container than what the parent system is running