SPL now handles the guarantee to separate ZFS's userland components from Linux's constant internal kernelspace API churn. ZFS, internally, doesn't really know what a Linux is beyond a few little bits here and there.
These changes were made years later, but the developer of ZFS-FUSE disappeared, so they were never used.
The FUSE v3 API further builds on that, and today, it's definitely possible to build a high-performance filesystem using FUSE. In fact, there are many examples of this in the wild: GlusterFS, Ceph, NTFS-3G, and so on.
Today, there's absolutely _no_ reason ZFS would not sensibly work as high performance filesystem entirely from userspace. In fact, it still remains an open issue and a desirable target for ZoL: https://github.com/zfsonlinux/zfs/issues/8
I'll admit, I'm all talk and if it really mattered, pull requests are probably welcome.