I think I found a bug in the book of heroes of rexxar’s

While I was fighting lady darkscale in the book of heroes of rexxar’s, I found that the damage of the boss’s skill increases when there is minion with spell damage at present. I know this is not how skill damage interacts with spell damage in standard games, but I’m not so certain whether this one is by design or not. Could anyone tell me have I found a bug or that is just how it is.