Actally both the Blenheim and Battle were considered "light bombers" (c. 1000lb bombload) in 1939.
The Hampden and Wellington were "mediums" (c. 4,000lb)
The 1939 "heavy" with 7,000lb load was the Whitley
What you have here is journalistic accuracy. (little changes over the years)
What? No way! Not in the New York Times. I just can't believe that... ;-)