It’s a firewall thing not a batting thing. You need a stateful firewall to do that kind of smart port forwarding. Which, to be fair, all consumer routers should have.

Stateless firewalls, however, need to have explicit rules for UDP traffic. So that’s what Nintendo are addressing here.

NAT functionality especially for UDP can be incredibly flaky in a lot of consumer hardware, mangling payloads, randomly dropping associations or having extremely short timeouts, and other plain buggy behaviour.