Wait, aren't you aware of the "It's not illegal if a billionaire does it" rule?
"I am going to dump 1/10th of the company that I very publicly acquired if they don't do what I want them to do" feels like manipulation since he's controlling the value of such a large amount of it.
But is it illegal to be willing to lose money if a threat isn't met?
Furthermore, would it be illegal for him to dump his shares, allow the trajectory to tank the price further, then rebuy when it bottoms, rinse and repeat?
No, you are allowed to sell your shares if you don't want them anymore.