SRX port forwarding clarification

Hi,

I am migrating from SSG to SRX: allow SSH on a non-standard port from the WAN and forwarding it to a machine on the standard port.

WAN:nnnn -> 192.168.3.100:22

Trying to understand why I need to do something that was posted in 2011 from https://forums.juniper.net/t5/SRX-Services-Gateway/SRX-port-forwarding-translation/m-p/100942. Specifically the untrust-to-trust policy needs to match the junos-ssh application in addition to my custom one that refers to the non-standard port. Isn't the destination pool host definition enough?

Thanks

version 12.1X46-D71;

security {
        destination {
            pool DNAT-host-SSH-test {
                address 192.168.3.100/32 port 22;
            }
            rule-set DST-NAT {
                from zone untrust;
                rule Rule1 {
                    match {
                        source-address 0.0.0.0/0;
                        destination-address 0.0.0.0/0;
                        destination-port nnnn;
                    }
                    then {
                        destination-nat {
                            pool {
                                DNAT-host-SSH-test;
                            }
                        }
                    }
                }
            }
        }
    }
    policies {
        from-zone untrust to-zone trust {
            policy UNTRUST-to-TRUST-test {
                match {
                    source-address any;
                    destination-address any;
                    application [ SSH-DNAT junos-ssh ];
                }
                then {
                    permit;
                    log {
                        session-init;
                        session-close;
                    }
                }
            }
        }
    }
}

applications {
    application SSH-DNAT {
        protocol tcp;
        source-port 1024-65535;
        destination-port nnnn;
    }
}