Constant, backbreaking work was not a feature of hunter-gatherer societies in the way it was of early agricultural societies, yes; at the same time, they still worked equal to or longer hours than we did, at things we would likely consider quite grueling and boring (mostly food processing), and what they got out of it was a level of nutrition even they regularly considered inadequate; moreover, a lot of the reason the average per day work estimate is so low, as the paper covers briefly, is that there were very often times, especially during the winter, where food simply wasn't accessible, or during the summer, where it was so hot it was dangerous to work, so there was enforced idleness, but that's not the same thing as leisure.