wan = outside firewall
lan = inside firewall
You don't want to bridge them because then it just bypasses the firewall.
lan = inside firewall
You don't want to bridge them because then it just bypasses the firewall.
LAN traffic is still going through the router, where the firewall supposedly runs.
Regardless of other devices having a public IP, if the router blocks forwarding packets (which is one of the main functionalities of a firewall), then the device(s) behind it are just as protected as if they had private IPs.
It's just a bit easier to setup forwarding as it doesn't require NAT, and it's also easier to open up by accident but having a public IP does not bypass the firewall.