Anxiety disorders and depression are NOT the same thing and should not be confused with each other.
Also, it's inaccurate to say that either are caused by a chemical imbalance. As reported by Harvard Medical School: "Research suggests that depression doesn't spring from simply having too much or too little of certain brain chemicals." It's much more complicated than that.
There are several things that are completely wrong with the WTBTS's approach to serious mental health issues. First, they should be directing people that suffer from depression, anxiety or other such mental health concerns to seek help from qualified medical professionals, not window-washers or janitors that went to a two-day seminar put on by the WT.
Secondly, many of the emotional and mental health issues that JWs suffer from are a direct result of the beliefs and practices of the religion. As many have posted, and has oft been observed previously, leaving this religion frequently leads to an immediate reduction in anxiety and depression. However, that is not always the case because there can also be other underlying causes which again should be addressed by trained and qualified mental health professionals.