"Confederates in the Attic" claim

On page 89 of Confederates in the Attic, our protagonist sees more of the southern racism complex. He enters a bar where the proprietor and a customer send messages through visual media. Here's the quote:
"Behind the bar stood a man in a polka-dot cap and a T-shirt adorned with a swastika. This was the proprietor, Redbone. He served me a beer and huddled with a man whose shirtfront proclaimed: I've got a Nigger in My Family Tree. The back of the shirt showed a lynching--a cartoonish black man, dangling from a branch."
I want to focus on the second man. The claim he is trying to make marks the absurdity of a black person being a part of his family, argumentatively, any family. This is derived from his humorous appeal towards the issue. Although the shirt may seem funny, the only humor in it is the fact that it is incredible (not to mention disgusting) that anyone would actually consider wearing that and joking about a lynching. His audience must be the other townsfolk who would also find it humorous, such as the man donning the swastika. It may also be a warning to blacks or people who disagree.
I almost feel like his appeal is using ethos. He establishes himself and his family as slaveowners, meaning he has the "right" to wear such clothing.
In that time period I assume the shirt would have been very successful indeed. Almost an ice-breaker between slaveowners. We can even relate it to risque T-shirts we have now such as ones that say things like "Rock out with your cock out" or even the company "FCUK" is borderline inappropriate. Just like these shirts get a huge response out of people now, that humorous shirt must have been accepted then. It's unfortunate, but humor and success almost go hand-in-hand. This also accomplishes its goal with the reader because it stands out and is so shocking.