Event horizon bug?

Based on the verbage of Event horizon, only chaos skills should be affected by the downside. If I understand support gems correctly, then they are not considered "skills". Correct me if I'm wrong there.

If all this is true then while I'm using "Temporal Chains" with "Decaying Hex" "Event Horizon" should not be effecting me with downside, yet it does. Is this bug or incorrect verbage or am I not understanding something?