I think it's important to understand that GULag was not only an instrument of industrialization -- which you're absolutely right, it was -- but that it was also an instrument of maximizing the utility, to the state, of enemies of the state, after which the lives were discarded.
Of course he's not the most accurate writer on the matter, because 1) his purview was necessarily limited and 2) he has an understandable bias. I do think that he's right in extending the concept of GULag back to 1918 or so even if it wasn't a legal statement of corrective labor camps; such camps seem to have been a tool of Soviet governance almost from the point of their seizure of power.
My point was just that labor camps and extermination camps aren't mutually exclusive, not in Germany, and not in the USSR. I wouldn't doubt that China has some similar sort of thing going on nowadays, either.