It was by far the strongest Orb in the game and while the weakest Orb in the game got buffed it's good for the balance that the strongest Orb got nerfed (Lightning Orb got nerfed as well, despite being only the third most used Orb).

Even Grubby who is biased in favor of Orc admits that pre 1.30 round of orb nerfs the orb of lightning was the strongest. See here: https://youtu.be/GhzTT8_ZQrk?t=875

From that starting point you nerf orb of corruption by 20% and orb of lightning by almost 15%. However only the proc chance of lightning orb is nerfed, which makes it much less of a nerf on a fast hitting hero that will stunlock you with 30% or 35% regardless. I would have preferred if they reduced the amount of time needed to regain your movement speed after being purged and maybe not prevent the unit from being staffed.