This statement is FALSE...
I have seen that statement multiple times and never has that been the case.
DuckDNS is like a phonebook, it has one job and that is to assign a subdomain name to an IP,
If my-ha-domain.duckdns.org resolves to your IP then DuckDNS is working and you should check elsewhere.
Verify this with nslookup on Windows (dig on MacOS/linux) or a webservice like https://network-tools.com/nslook/
Example from nslookup:
C:\Users\user>nslookup my-ha-domain.duckdns.org
Server: a0d7b954-pi-hole
Address: 192.168.2.200
Non-authoritative answer:
Name: my-ha-domain.duckdns.org
Address: 127.0.0.1