What matters under the Copyright law (and thus the GPL) is whether the module is a derivative work of the Linux kernel or not.
ZFS was originally created for Solaris, and works on multiple operating systems. So ZFS itself is obviously not a Linux derivative. If the original ZFS could be directly linked with the Linux kernel without modifications, it still wouldn't be a Linux derivative.
But ZFS had to be modified to work with Linux. It can be argued that those modifications are Linux derivatives. We haven't had a definitive ruling on this yet.
ZFS from Solaris / BSD --> not a Linux derivative, even if it was directly linked into Linux.
ZFS with trivial modifications to work with Linux --> not a Linux derivative
ZFS with extensive modifications to work with Linux --> judge's ruling required
The only reason that linking matters is because Linus's statement that binary modules are OK would have some weight with the judge. However, Linus is not the only copyright holder of the Linux kernel, and other copyright holders have disagreed with Linus on this statement.
It's a Linux kernel module running in Linux kernel address space, I'd say there is reason to assume it can be considered a derivative work, and thus a license incompability.
Do you think that there is reason to assume that every program that ran on MS-DOS on an 8086 was a derivative work of MS-DOS? The programs and MS-DOS all ran in the same address space on the 8086.
ZFS was originally created for Solaris, and works on multiple operating systems. So ZFS itself is obviously not a Linux derivative. If the original ZFS could be directly linked with the Linux kernel without modifications, it still wouldn't be a Linux derivative.
But ZFS had to be modified to work with Linux. It can be argued that those modifications are Linux derivatives. We haven't had a definitive ruling on this yet.
ZFS from Solaris / BSD --> not a Linux derivative, even if it was directly linked into Linux.
ZFS with trivial modifications to work with Linux --> not a Linux derivative
ZFS with extensive modifications to work with Linux --> judge's ruling required
The only reason that linking matters is because Linus's statement that binary modules are OK would have some weight with the judge. However, Linus is not the only copyright holder of the Linux kernel, and other copyright holders have disagreed with Linus on this statement.