In the article they consider high volume workouts to be heavy workouts. Shorter rest periods relate to lower volume, of a 45 second workout may lead to more lighter sets, but lower volume.
That's why they conclude that workouts with 5 mins rest between heavy sets lead to higher volume, strength and muscle hypertrophy.
Please don't bite my head off if you don't agree with this. This is how they sum up volume in the article.
For me, the whole concept of volume is meaningless. I rest long enough to give me the best compromise between strength and time for each set. The length of time I rest depends on how heavy I want to go for the next set and how well I've recoverd from the previous one.