Why exactly is Thor so against this initiative? Is it purely on "nepo baby" mentality alone, and/or because he has some holdout belief that blizzard will rehire him?
He certainly doesn't want Blizzard to rehire him, given that Heartbound features a hellish office level as one of the first (and so far only) levels in the entire game. He hated working an easy nepobaby office job so much that hell is how he views office work and Blizzard now.

(
video sauce)
Remember how fast he jumped to conclusions with that Apex Legends remote code execution hack live on stage? Doesn't matter that he got everything completely wrong. As soon as something pops into his mind, that thought immediately becomes the truth to him and he has no qualms about vomiting it out to millions. Why? Because narcissists
fucking hate having to save face. They
fucking hate being proven wrong. But at the same time, they love
everything that
they can possibly attribute to themselves
so deeply and innately. Combined together, they invent bullshit like it's nothing.
So, with SKG, he ended up doing the same thing again. Glance at something for five seconds, do zero research, don't process a single word that anybody is actively saying, then immediately invent a conclusion that makes him look good. People tell him "isn't this tidbit incorrect?" and his ego says "eat my entire ass" out of sheer reflex. Now that it's all biting him in the entire ass, what does he do? Quintuple down like the lolcow he truly is instead of admit "sorry, I misunderstood."
Narcissism is your answer. Deep, deep, deep, deep, deep down in his mind, he cannot sincerely admit fault. He's clearly been like this his entire life, and he'll certainly continue being like this for the rest of his life. Now that's funny.