WEll, I believe the poster said we WERE too nice to say so - remember? We are nice folks - but yes, there is a certain 'tightening' that occurs when we hear some 'nawthurners' speak. It is just instinctive and comes from years of being derided and make fun of by those folks who are aptly described in the article as 'yankees'. After we get to know you, if you are good folk, it's not so bad. Usually though, we do make some attempt at teaching you to speak properly.
I remember a time when my husband was in the military, (late 50's) and for a couple of years after, we met plenty (for me) 'yankees' per the description and many,not all, thought all folks from the South were ignorant, uneducated, backward. It was just their absolute ingrained belief - before they left Texas many had been won over - one way or another. There's kindness and graciousness, assimilation (marrying one of those TExas girls) or if necessary, - other ways.
Well maam, I did have to learn to speak much slower and to pronounce my "R"'s just to be understood down there. After being back in NY for 17 years now, I will admit to sliding back to my NYese.
I haven't lost my love for that wonderful southern word "ya'll" though.