They mean not to use it as your primary DNS server. Pinging it and using it for ...

orofino on Sept 22, 2013 | parent | context | favorite | on: 4.2.2.2: The Story Behind a DNS Legend

They mean not to use it as your primary DNS server. Pinging it and using it for temporary DNS is likely fine, I wouldn't suggest statically assigning it for permanent use though.