Not really. The non-addressability comes from not globally routed ("private") addresses on the "internal" network: They may prevent someone on the other side of the planet from reaching those "internal" hosts on your local network, but that's orthogonal to NAT: You can do NAT between globally routed addresses (which thus would be reachable directly ... unless there is a (stateful) firewall preventing that!), and also, just because those addresses aren't routed globally, doesn't mean your ISP (or whoever is connected to the "outside" link of your NAT gateway) couldn't send you packets directly addressed to your internet network that your router/NAT gateway would just forward to your local network (once again, unless you have a (stateful) firewall that blocks those packets).

Now, it so happens that dynamic NAT also needs to do connection tracking in order to be able to map addresses back and forth, just like a stateful firewall does, and that therefore, it's easy to also implement stateful packet filtering on top of the same connection tracking state - however, there is no need to do NAT in order to do the connection tracking and the filtering based on that, you could have the exact same stateful packet filtering with the exact same security properties, just without messing with the address fields of the packets and all the bad things that result from that.