Was V.S. Naipaul correct that "the buckle of the Bible Belt” contains mixed metaphors?

It doesn't meet the strict definition of a mixed metaphor, but the term "mixed metaphor" in popular use has expanded to refer to any situation which involves yoking together terms from different contexts or combining a metaphor with a literal term from a different context.

Wikipedia says "Bible Belt" was coined by H L Mencken in 1924 to refer to a region of the US. A sense of "belt" meaning "II. A relatively long and thin or encircling region of sea, land, sky, space, etc." goes back to the 17th century, but Mencken's usage most closely approximates this OED definition, common since the second half of the 19th century.: "b. With preceding modifying word. A zone or region of distinctive character; spec. one which is notable for the production of a particular product. Chiefly U.S. in early use." (Source: OED, Belt, n.1)

Hence "Bible Belt" isn't really a metaphor, it's an example of a commonly used sense of the word "belt".

"Buckle" in the quote is a metaphor: if you think about the functions of a buckle, it's something in the middle, or that ties a belt together, as a city may unite a region or provide connections, but it can also be a decorative thing, with belts with large, decorative buckles often associated with a simple American folk culture (particularly in cowboy or western wear). I'm not sure exactly what the authors intended, but either way it's metaphorical.

But what is a mixed metaphor? Merriam-Webster defines it as "a figure of speech combining inconsistent or incongruous metaphors". This would seem to require two different metaphors. As I've shown, "buckle in the Bible Belt" doesn't really contain two metaphors, but only one ("buckle"). However, while the definition suggests two metaphors, in practice the term "mixed metaphor" is often used more loosely, for instance when a metaphor is incongruous with the rest of the sentence, often occurring when a word has multiple meanings.

Merriam-Webster's main example is "If we want to get ahead we'll have to iron out the remaining bottlenecks". In this case "iron out" might be a metaphor, but "bottleneck" is a common term for a point of blockage that has long since passed from being a metaphor to being an ordinary meaning of the word.

They also have a section "Recent examples on the Web" which includes "Eye strain, like neck, back, or wrist strain, is nothing to be sneezed at, to use a very mixed metaphor." In this case, you have one metaphor "to be sneezed at" (although again it may be moving from metaphor to ordinary meaning) and something which definitely isn't a a metaphor (strain in various parts of the body).