He's suggesting a tweak to Kayle. I'm going to assume you're been reading other topics on Kayle, and caught my comments that Intervention being able to shield allies has lead to Kayle being quite team-dependent.
However, 10-20 base attack damage does not make up for changing Intervention so drastically. Back when Intervention was self-only, it was a flat 4 seconds with a cooldown of 60/50/40/30/20. It was also her E ability; Righteous Fury was her ultimate (I forget the exact specifics, but it had a much longer duration back then). Kayle got nerfed a lot when they made Intervention able to target allies because they nerfed all her other skills to make up for it (Riot has admitted that back then, Kayle was a "lopsided" champion). Your "tweak" would just drive Kayle down into the depths of "never heard from again."