Well Jake, if you say it's rubbish then it must be rubbish, so I surrender. You obviously have no love for Dan Brown's writing and neither do I; so, it seems, there are things we agree about.
Are you sure that the use of semicolons is justified? Should phrases, as connected, independent clauses are, be separated by semicolons? 
My take on the error, or, rather, errors of construction is not about that though; I play fast and loose with punctuation all the time. My point is that there is no referential connective in any of the phrases following the second clause so each is just left hanging in the air. Additionally, each one expresses a different facet of stravaiging yet the sentence concludes with the singular terminal phrase, "in itself paramount", again with no connective. Admittedly the construction is wholly technically adrift and sloppy but the meaning of the sentence is clear to me and as the expression of thought untrammelled by syntactic function I find it appealing; I don't think syntactically.
I have to admit that your own example is a real humdinger. 
I was only venturing my opinion, as indeed were you, although I do think the plain facts of Jim's prose back me up.
As for the use of the semi-colon, just my guess or "; just my guess"?. I was educated in the comprehensive system in the 1970s where the rules of grammar and punctuation were not explicitly taught, so I have no expertise in this area. Semi-colon or comma? It doesn't really matter, does it? As I say, the real problems of Jim's prose go way beyond this.
The real tragedy may be that Jim really has something of value to say but that the message is lost because of complexity of the writing. He really could use an editor.
Still, at least we've got a love of Dan Brown in common
. Google Professor Pullum for a very entertaining blog featuring entries explaining just why Dan Brown's writing is so bad