And about thirty seconds worth of looking into the character himself would reveal a lot more than what you've just begun to assume.
The expansion isn't about Thrall and Malfurion. While they may play a part in it, the expansion is about Deathwing and his rampage on Azeroth and the effects of it. Just because Malfurion appeared again at this time doesn't mean he's the star of the show, nor does Thrall's decision to take his place as the World-Shaman make him the King Bee.
When it comes to Hamuul Runetotem, there's plenty that puts him on the same ground as everyone else:
* He's the Arch Druid of Thunder Bluff, and a leader of his people.
* He is the highest ranked Tauren in the previously Night Elf dominant Cenarion Circle, and is formally recognized as the first Tauren druid in nearly twenty generations.
* He is a student of Malfurion, having sought him out himself and befriended him.
* He is a member of the New Council of Tirisfal, standing in as the druid representative after it was deemed necessary to have a practitioner of other arts, with Malygos hampering the arcane.
* He discovered the World Tree was going insane, and it is he who discovers Fandral's duplicity.
* He remained in the waking world to protect the others, including Malfurion, in their battle against the Emerald Nightmare, defending them against the nightmare apparitions that appear.
That alone is enough to see why Malfurion regards him as an equal, but I'm going to press the point a little harder. As much as you want to venerate Malfurion, it's only fair to remember some other small, but important, things.
Malfurion did not bring Archimonde to his knees. He blew a horn that called on the the very spirits of the night elves to destroy the demon. If anything brought Archimonde down, it was the very will and strength of nature. Malfurion only called on it, and he would not have had the time to do so if Archimonde had not been stalled by countless other people, most of whom have juuust about as much screentime as Hamuul.
And while we're on the topic of Malfurion and all of that? When have you ever known Malfurion to be a spot-light hogging, high and mighty, xenophobic jerk who can't treat others as is their due? He owes his life to Hamuul in some ways, and knows the Tauren is a respected ally and friend.