Nit: the QotD example is missing a systemctl start fortune.socket (tested with systemd 245, per current Ubuntu LTS).
systemctl start fortune.socket
It is a bit annoying that inetd stuck to fd 0 for wait-mode stream services, requiring a FD song-and-dance to adjust to something more “normal” for each connection. DJB got this right, IIRC, though I never actually wrote to that API.
Thanks! You’re quite right. Fixed now.