I recommend you use autofs rather than fstab. What I do is mount all remote shares in /mnt/ on OSMC and then in Kodi add those sources through the ‘root file system’ option.
See File sharing with a NAS, media server, windows share or other device and Mounting network shares with autofs (alternative to fstab)
The instructions you linked are a bit dated. The modern way to manage nfs-server is via systemd:
sudo systemctl enable nfs-server
sudo systemctl daemon-reload
sudo systemctl start nfs-server