This is correct! Depending on the app stack, you can either bind to something like :: to listen on all IPv4 and IPv6 addresses, or bind specifically to the 6pn address.
Binding to either _local_ip.internal or fly-local-6pn will also work for some runtimes. It’s wildly inconsistent. You can test the latter by adding an entry to your local/etc/hosts and running your app locally.