However, Mr Wu added that attackers would have to get access to the network first and then guess the admin password.
That sentence doesn't seem to be in the article? Was it in there and they've deleted it?
The article suggests the password is not needed as the vulnerability is you can access the router bypassing the security. If the password is needed then it is not a security flaw as such as they were just changing DNS settings which is something you can do if you have the password.