usenet is decentralized at server level. In theory all the servers have the same content. Currently everything is obfuscated because of the take downs - so you need an indexer (the users upload the nzbs to that website. Think of a nzb like a .torrent file.)

I have ports forwarded locally to deal with my router, that doesn’t apply to my ISP down the line.

So you are under CGNAT: https://en.wikipedia.org/wiki/Carrier-grade_NAT

From the wikipedia link:

Carrier-grade NAT usually prevents the ISP customers from using port forwarding, because the network address translation (NAT) is usually implemented by mapping ports of the NAT devices in the network to other ports in the external interface. This is done so the router will be able to map the responses to the correct device; in carrier-grade NAT networks, even though the router at the consumer end might be configured for port forwarding, the “master router” of the ISP, which runs the CGN, will block this port forwarding because the actual port would not be the port configured by the consumer.[7] In order to overcome the former disadvantage, the Port Control Protocol (PCP) has been standardized in the RFC 6887.

You will only be able to upload to someone with a public ip address or with UDP hole punch (but there are some technicalities - and doesn’t work in all the cases). The same will happen if you use torrents. Maybe you don’t notice difference because of the sheer amount of people using seedboxes and vpn.