Why?

I'm trying to put what I've learnt about English grammar and punctuation to the test by reading Eats, Shoots & leaves, and studying how the sentences have been put together.

I've found a few items where I'm not 100% sure I've got it right, and I want to check I have interrupted it correctly.

A couple of weeks after Eats, Shoots & Leaves was first published in November 2003, I met an old sub-editor friend at a party who said: "You've written a whole book on punctuation?

Now A is not a subordinating conjunction so the comma isn't there because a sub clause is being joined to a main clause. So I thought maybe this is a case where non-essential information (like a weak interruption) has come at the start of the sentence so we can only see one of the bracketing commas.

I met an old sub-editor friend at a party who said: "You've written a whole book on punctuation?

Can stand alone and make sense without 'A couple of weeks after Eats, Shoots & Leaves was first published in November 2003'

But there is the find A couple of weeks after Eats, Shoots & Leaves was first published in November 2003 doesn't stand alone so is is a sub clause and if so how?