> Correct me if I'm wrong but in case of IP_BIND_ADDRESS_NO_PORT doesn't the unique 4-tuple (sourceip+sourceport+destip+destport) limit still remain?

Yes, it still remains.

> As you only defer/delegate to kernel to assign the ephemeral port in connect() rather than at the bind() time (when the destination is not yet known) so in case of a single source ip and single backend/port the ~65k limit still exists.

You are right for the case of single source IP and single backend-port pair.
The thing is that in case of single source IP and multiple backend-port pairs overall amount of connections is still limited by 65K. Linux's IP_BIND_ADDRESS_NO_PORT increases the limit up to 65K connections per single backend-port pair (single source IP remains the same for all connections to all backends) and NGINX supports the feature. So I wonder if there's something like on FreeBSD.