This appears to be inaccurate. They appear to be distributing it as a binary module, outside of the kernel tree, just like many other binary-only kernel modules.
The binary kernel image is a separate issue from the kernel packages (deb); they could include multiple files in a kernel package (deb) that are licensed under different licenses.
See zfs.ko here: http://blog.dustinkirkland.com/2016/02/zfs-is-fs-for-contain...
"However, there is nothing in either license that prevents distributing it in the form of a binary module or in the form of source code.":
http://zfsonlinux.org/faq.html#WhatAboutTheLicensingIssue
The binary kernel image is a separate issue from the kernel packages (deb); they could include multiple files in a kernel package (deb) that are licensed under different licenses.