The above quote appears to be a good example of argumentum ad verecundiam...
It doesn't appear to represent that fallacy well.
An argument from authority is that X must be true because Mr. Y says so.
The argument here isn't that Oath Keepers are "true", but that they aren't a group of icky camo wearing embarrassments. That is directly responsive to the charge that the group is a bunch of icky camo wearing embarrassments.