The mounts and achievements tied to the Siege of Orgrimmar raid are being handled differently than those from past raid tiers.
achievement only requires players to complete non-heroic achievements. The devs weren't really happy with how the Glory of the Raider mounts were playing out in recent tiers, and decided to change how they worked. They mostly ended up being earned by a subset of the people who were fully clearing the zone on heroic already. People typically would finish killing all the bosses on heroic and then take a week off to kill them on normal mode for achievements--which made raiders who disliked achievements unhappy that they were missing out on loot upgrades.
When the Glory of the Raider mounts were initially created, the Normal/Heroic issue wasn't a problem because Naxxramas had no heroic difficulty (although, the devs aren't fans of some achievements like the Thaddius one.) However, problems started in Ulduar, when you had to decide between doing something like 弗蕾亚
+3 or or achievements like 秋风扫落叶（25人）
With Flex raiding being a separate lockout, those players who really like achievements can get them done on this easier difficulty level without affecting heroic progression or loot acquisition. And since Flex can handle any number of raiders, you don't have to worry about leaving friends out if an odd number of people like the achievements.
Heroic Garrosh will also have a 100% droprate on a special mount, which will be nerfed to a 2% droprate when the next tier comes out. This system for heroic mounts has been used previously for rewards like 无敌的缰绳
but hasn't been used in Pandaria. The devs realized that while there were some cool raid mounts like 星光云端翔龙缰绳
, they were all RNG based and didn't reward players that were good or persistent. So rewarding players for accomplishing something really special felt right for Tier 16.