I have the same issue. I dm'ed Tyber and he says he will get back to me in this regard. In addition to not providing the +30%, It seems to disregard the the elemental rune chosen and switch it back to shadow dmg, regardless of the rune you use. So I am not sure if it is truly a -30% it just doesn't take into account the chosen rune. I did not test with no rune which I might do shortly. However the wording is brief and I am not sure if slicing dash is worded as they intended. Yes as I suspected no rune you do get the 30% damage increase. So it isn't a true minus damage, somehow they don't adjust for the rune choice and increase the damage based upon that rune.

https://ibb.co/51cspLp

https://ibb.co/sWdXwv0