The whole post is on base on the fact that "TW" in their URL. But I think this is really just a internal implementation detail of Google Translate, which is not intended to display to users (e.g. complete hidden on their app). Everywhere on their UI shows "Chinese (Traditional)". That is what they try to communicate with users.

Sure, zh-TW is somewhat misleading. But they nor say that parameter is a ISO 639 or RFC 5646 conformed.