Yeah it’s a common trip up. We’re all so used to the way that things are done in IPv4 that our natural response is to try and apply IPv4 logic to IPv6, but you’re absolutely right.
Many people think NAT is a security feature but but that’s only a coincidence and it doesn’t do anything a firewall doesn’t already do. And if we take it one step further we can actually see that a firewall and IPv6 is actually more secure than NAT. The only inherent risk of port warding in NAT is that the IP you’re forwarding to is ultimately arbitrary. Think, have a port open to SMB for a publicly accessible file sharing container, then later ditching it and via DCHP your laptop picks up that old IP and now voila you’ve technically exposed your laptop. It’s not quite that simple but that’s the essence of it.
But with IPv6, IPs are no longer arbitrary. When you allow access in certain ports to a certain machine and that machine goes away, that rule will always only allow access to nowhere.