I don't think its a nerf. Different skills use different coefficients of the heroes magic or skill attack. In addition to Horns of Hell, Swift Prye and Warcry are examples of other skills/attacks that have a damage coefficient of less than 1. Thousand strikes has a damage coefficient of approximately 1 （hard to tell for sure for skill attacks, because there is a damage multiplier based on physical attack as well), while skills like Driving Blow, Rallying Strike, and Piercing Thrust have damage coefficients of greater than 1.