Debate and all is a fun exercise! Oooh! Challenge accepted. Too late.
Follow me into this rabbit hole...
Okay here it is.. in terms of scholarly matters, sign Language has been proven to be a legit language through scientific analysis by language scholar William C. Stokoe a while back. Sign language not being valid as verbal or language is just a part of a stereotype that is really unproven, biased, and is harmful to those that depend on it 100% in terms of being considered for jobs and other things. I base being verbal solely on ASL being a legit language to use as a way of communicating. Some people do not view it as a gestural language, even though hands and physical expressions are used to convey meanings and context instead of the voice/pitches. Gesturing to me is "pointing at things" and playing charades so I don't think that has a valid application on a legit language like ASL. Gesturing involves a lot of guesswork. ASL does not. Ya know?
If we account for the ones who created the stricter definition of "being verbal," we can assume the bias of "Disability Privilege" was used. In this context, I mean the victor determines how history is explained in the history books to school children.
Here's an article on Stokoe's position of sign language regarding "verbal" and "speaking" - it is not peer reviewed and a bit old, though. I find this article interesting all the same. You can google plenty of material on Stokoe and his research
http://www.nytimes.com/2000/04/11/us/william-stokoe-jr-sign-language-advocate-dies-at-80.html