The notion I get from the article is that security becomes a huge problem when every node is exposed to almost every other node by design intent. That's why NAT is mentioned several times.
It's very very easy to replicate the filtering behaviour of NAT for situations where its being used that way. Simply block connections into the network that weren't initiated by clients in the network itself. Every stateful firewall can easily handle that and it doesn't come with the security loopholes of NAT.
...which a lot of people haven't, because they falsely believe that NAT by itself is enough to stop all inbound connections to their network.