fly proxy specifically connects to your app via our private network and it’s all IPv6. There’s no intermediary involved (except our wireguard gateways), so your app has to be listening on the fdaa:-prefixed IP. Easiest way to achieve that is to bind to everything (w/ [::]).
If you’re accessing your app via the public hostname we provide, then that goes through IPv4 (the IP starting with 172. in your VM).
So to answer your question: yes only listeners bound on ipv6 are accessible via the fly proxy command.