The document is fine, I updated it this year after the switch to nftables in Qubes OS 4.2, it works fine for me.
I made a forum post about forwarding Qubes OS 4.2 nftables / nft firewall guide and also I wrote a simple script to setup forwarding for a given qube, to run from dom0 [Qubes OS 4.2] Easily NAT qubes port to external network