The inbound firewall rules only apply to the following types of traffic:
Inbound traffic that is defined in Port Forwarding
Inbound traffic that is defined in Inbound NAT Mappings
So, no inbound traffic is allowed if Port Forwarding or NAT Mapping is not defined even the default inbound rule is configured allow by default.
Some deployment will use our device as link load balancer and a firewall is behind our device. So, the administrator just needs to plug and play with our device by having the default inbound rule that allows any traffic.