I’ve used HA and installed duckdns before and have been able to access my HA externally outside my home network. Now I did a fresh install of HA and installed duckdns , but I haven’t configured duckdns neither started the service, but I can still access my new HA installation away from home.
Because your duckDNS dynamic DNS service is still active (on the internet), your security certificates have not expired, your public IP address has not changed and your router still has the port forwarding rules set up.
It will eventually stop working when the either the certificates expire or your public ip address changes. Unless you start the addon. It is responsible for renewing the certificates and updating any public IP address change.