Your accusation that I am verbose, voluble, long-winded, and loquacious is completely unfounded. I've never uttered a sentence in my life that contained more than twelve words. - The previous argument provides an example of___.

inconsistency

Explanation:

In argument or logic, inconsistency often referred as internal contradiction or logical inconsistency is a term of a fallacious argument, whereby  two or more propositions are asserted that cannot both possibly be true.

Hence, in this case, being described as voluble, verbose, long winded and loquacious is contradictory to being described as an individual who has never utter more than twelve words sentence before.

Therefore the argument is rather considered as Inconsistency