It only works with SMB in actual versions of macOS. But it insist of using AFP+ as its Filesystem.
It doesn't look like it works even between macs via SMB to me.
Mind, the laptop is on 10.14 and the desktop on 10.13, so that may be a problem.
(Assuming you mean APFS) Not true. I have Openmediavault running on my raspberry pi 4 and the two drives i use for dual-backup are both ext4. (can really recommend RPI4+OMV for time machine btw)
I suspect that's because the Time Machine solutions for NASes create a sparse bundle that has a HFS+ (or APFS, I'm not up-to-date) partition within.