Simply put, Kline uses Ancient Near Eastern (ANE) sources in order to further the interpretation of the Old Testament. The doctrine of Scripture itself, however, comes from Scripture alone, for Kline. In his book, The Structure of Biblical Authority, for instance, Kline has these things to say:
The formation of the canon, rather than being a matter of conciliar decision or a series of such decisions with respect to a preexisting literature, was a divine work by which the authoritative words of God were through the mystery of inspiration inscipturated in document after document, the canon being formed by the very appearance of these God-breathed scriptures…Along with the performance of this apologetic-critical function, orthodox scholarship has addressed itself to the more positive study of the canon. Efforts in this direction, however, have been largely concerned with the proper formulation of Scripture canonicity in the dogmatic categories of the Bible’s own objective self-authentication as word of God and the Holy Spirit’s internal testimony to the Word…It is necessary to insist constantly that the Scriptures, whether the Mosaic covenant documents, which constituted the nuclear Old Testament canon, or any other Scripture, are authoritative-uniquely, divinely authoritative-simply in virtue of their origin through divine revelation and inspiration. Certainly, then, their authority as such is not to be accounted for by looking beyond them elsewhere (pp. 23-25, 37).
The last quotation is the most important, because there Kline, in the context, explicitly deals with the place of ANE treaty documents in our understanding of Scripture. He says that ANE sources provide a “purely formal” cast of canonicity. In other words, Kline clearly distinguishes between the authority of Scripture, which is derived from Scripture itself (see all of page 37 for this crucial distinction), and the form of Scripture, which bears resemblance to the ANE treaty documents. Its authority as the word of God does not depend on ANE sources. This is the crucial contrast with Enns, who wants our very doctrine of Scripture itself (what Scripture as a whole is) to be defined in reference to ANE background documents.