NAT overloading private IPs onto a single IP (what 99% of consumer routers are doing) is an implicit firewall. Without additional protocols like UPNP or manual configuration of port forwarding, nothing behind this style NAT can receive unsolicited traffic.
“Mangling routing” to a single source IP requires state tracking of outbound sessions to know where to send inbound traffic. No outbound traffic, no session, no inbound traffic.
“Mangling routing” to a single source IP requires state tracking of outbound sessions to know where to send inbound traffic. No outbound traffic, no session, no inbound traffic.