I kept thinking about this, because it bothers me for a number of different reasons.
I don't write fiction; I never have. I write other sorts of things, and I have many years experience teaching English lit and comp classes, among other things.
* A "second-rate" Shakespearian writer is like a silver-medal winner. It's not a bad place to be.
* Hack in the context of writing is used as a sneer by "artistic" writers to writers who write for money; a hack is a writer-for-hire (
s.v. AHD Hack 2 verb intr).
* “doesn’t understand his own lines” is not really sensible in the context of
hack. Hack clearly refers to a writer; yet “doesn’t understand his own lines” sounds like a reference to an actor.
* It’s not something you would say to a writer. Most writers are producing prose. Poets write verses. Even playwrights typically produce text of some sort (play, script, scene, etc.). The only people who think of
lines are actors.
* Even so, it's a genuine commonplace for a writer or poet to suggest that the
meaning of their words is up to the reader; see for instance the anecdotes about Robert Frost the meaning of "Stopping by the Woods on a Snowy Evening." Writers don't understand the meaning of their works because meaning depends on the reader.
So yes, this is a very odd thing to say. It's mean, but it's also not sensible.