"Ensida didn't evade out adds. They just M/Ced something for a buff."

This is key. Ensidia didn't trivialize any of the mechanics of Hard-mode Hodir when they did the Flower Power thing. They still had to contend with Biting Cold and all the runes and buffs and the helper-NPCs, and they still had to work their butts off to pull the kill off.

By contrast, Exodus evade-bugging Guardians removed (1) having to pick out, taunt, and pull 20 yards the marked Guardians from a crowd of 20 guardians within 10 seconds, (2) having to heal 2 off-tanks holding 5 100% guardians (each hits for 20k-25k) every 45 secounds with a limited number of healers (I read that 4 is the current strat), and (3) pulling off enough DPS to get those guardians back to 1% quick enough to have time to get back to the boss. Exodus's exploit turned Yogg-Saron into a spank fight (don't even need a tank).

That's the difference to Blizzard. Additionally, Blizzard probably realized that buffing Hodir in the middle of the race was a bad idea, especially since it was done after raiders got it down for the lockout. But I wouldn't be surprised if Blizzard's more concerned about exploits that remove mechanics than exploits that make the numbers easier. That's why they hotfixed Freya-zerg, and that's why spell-stealing a buff that lets you ignore Naxx mechanics was bannable, but spell-stealing a +DPS buff was not.