That's something those western southerners told me. I don't know if a linguist would agree, but that seems to be the understanding of some actual language users...
All I know is that there is a second boundary somewhere through TX, NM, and AZ, because I've never met a native Californian who would say "y'all" non ironically.
When southern people say y'all to one person, they're really addressing you and your family (even though you might be the only one there.) If I ask "how y'all doing?" I want to know how you and yours are doing.
I just want people to stop asking me how I'm doing if they don't care.
It took me an embarrassingly long time to figure out that "How's it going" is a greeting, not an interrogative, and I want that change undone forever.
I just use "Howdy".