Yea, people consider NAT a firewall, but at best it stops direct connections from outside. People use this as a rationale to non secure individual devices on the network. Then the moment a single device on your network is compromised (do you really trust that Chinese IOT device?) every host that doesn't have its own firewall is at risk.

With IPv6 you at least say "Holy crap, anyone could connect to this, I better secure it from outside and inside attacks" which is how actual security works.