Yes, there is a difference between so-called "authentic" Mexican food and "Tex-Mex". However, Diana Kennedy didn't coin the term. The reason I know this is that I grew up in Texas and the difference was well known here long before Ms. Kennedy wrote a book (presumably for non-Texans?) about it. The term Tex-Mex has been around for at least six decades.
It, much less commonly, may refer to a dialect of Spanish which is sometimes called "border-lingo" or "border Spanish", which differs noticeably from the Spanish typically spoken in, say, Mexico City.