If you had you would surely not be suggesting they are the same type of drug as Xanax or Valium.
I'm not suggesting that antidepressants are necessarily "the same type of drug" as Xanax or Valium. Many classes of drugs (from a chemical structure standpoint) can have therapeutic indications for various conditions. No one type of drug is an antidepressant; nor are Xanax or Valium
necessarily efficacious against a single condition (italics indicate that they may be most efficacious against a single condition, but as an illustrative example, many drugs have therapeutic benefit in a variety of conditions).
All I'm really trying to demonstrate is that benzodiazepines have shown antidepressant activity/efficacy:
http://www.karger.com/Article/Abstract/406244
http://www.sciencedirect.com/science/article/pii/0006322395000497 (Note that alprazolam, which is indicated here as a possible option for those cases where other antidepressants are contraindicated, etc., is Xanax.)
This paper indicates that benzodiazepines have been used in conjunction with antidepressants to reduce anxiety and possibly increase efficacy of the antidepressant (in the interest of full disclosure; note that anxiety can no doubt lead to depression, so an anti-anxiety drug could also be viewed as an "indirect" antidepressant):
http://europepmc.org/abstract/med/11869584
The age of these articles suggests that, in the context of this thread (the 1970s), benzodiazepines may well have been prescribed as antidepressants. There's enough evidence for me to decide that this is a reasonable conclusion; especially in contrast to being certain that it is unreasonable.