Seems like a nothingburger article. Doesn't explain how to enable it, it just points you to the official docs.

I had a flat LAN and used Proxmox's firewall until recently. Then I finally set up my network properly with VLANs & very restrictive firewall rules and turned the Proxmox firewall off.

I've left the Proxmox firewall off figuring it would be simpler to rely on the gateway to do that type of thing rather than having two layers of firewalls, but I'm not sure if that's recommended.