The issue that HAProxy is encountering with SO_REUSEPORT is not an issue for Nginx's use case.
In the HAProxy case one of the sockets belongs to the old server which is going away, and a new connection can be assigned to that socket right before it does so. For Nginx, all of the sockets are accepting new connections.
In the HAProxy case one of the sockets belongs to the old server which is going away, and a new connection can be assigned to that socket right before it does so. For Nginx, all of the sockets are accepting new connections.