Why is pure damage especially dangerous to strength heroes?
What the commentators were actually referring to was the was one of the secondary effects of Timbersaw's first skill, Whirling Death. The effect is that if an enemy hero is hit, their primary attribute is reduced by 15%.
Since each point of strength is equal to 19 maximum hit points, Whirling Death effectively deals an extra .15 * 19 * the hero's strength points of damage to Strength heroes.
Source: http://dota2.gamepedia.com/Timbersaw