The general point is that being able to control the parent domain doesn't necessarily mean you control all possible subdomains as well. You need to prove ownership, not just control. Here's the relevant bit from the SO answer:

> If I have ownership of the parent domain example.com then I can freely create and control anything as a subdomain, at any level I choose. Note that here "ownership" is distinct from "control", which is what is validated by the ACME protocol.