If it was that case that we thought of a novel as no more than a sequence of words strung together in a particular order, then copying such a sequence would clearly be plagiarism. However, literary narratives are much more than simply sequences of words strung together is a particular sequence.
I haven't read the novel (not sure anyone else who is commenting here has either), but I can imagine circumstances under which such "mixing", as the author calls it, is a valid literary strategy. Using an analogy from the world of visual art, we can appreciate Duchamp's Fountain as an aesthetic object, or we can dismiss it as simply a urinal that Duchamp didn't even make - he simply took someone else's urinal and signed it R Mutt. On one level this is self-evidently true, but if we think the artwork is entirely described by this characterization we miss something important about the urinal as an artwork.
I'm not arguing that the "mixed" novel has any literary value whatsoever - but I am suggesting that we cannot decide that a priori simply on the grounds that it uses sequences of words arranged in a certain order which are the same as someone else has used.