I particularly don't buy it because ZFS used to have a FUSE build, and I'm pretty sure there's at least one company still running it in userspace in some form (something for k8s, IIRC?)

Maybe they meant that they couldn't separate out the various layers of ZFS into their own services?

I guess? I'm not personally convinced that even 1 ZFS service per zpool would be a serious problem, but I can see why it would be considered unideal to a microkernel designer.