NAT is not a security measure at all. It just obscures what's behind a firewall, but that is leaky and not reliable from a security perspective. It might make you feel better, but that is not security.
If I have an IPv6 router, I can miss-configure it in a way where all of my internal communications between IoT devices work as expected, but they also have discoverable addresses on the internet. This would give the firewall something to do, but I'd rather there be no route in the first place.
Also, if I trusted myself to properly configure my router for IPv6, I would put all of my IoT equipment on ULAs, which much like an IPv4 NAT would leave me with nothing to configure in the firewall.
If I were to take your claims at face value, using GUAs with packet filtering is far more reliable and secure than ULAs, and that seems preposterous.
A properly configured firewall for sure adds security, but isolation always wins out.
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.