Well, it has been 2 years. Heck, even if he sold at around 3rd or 4th month of this year, the profit would've been more or less the same. He actually lost out since if he sold by then, bought again when Bitcoin went down to $4k, then sold again now, the profit would be twice or more of what he has right now. Though you can't really say that this would happen back then, so it's understandable why he couldn't/didn't do as such.
I'd do the same if I were him, it's been almost 2 years so why not taking my profit out of it.
Investors like him could have a target price range and volume, possibly he sees it is reaching his target so he sold it at $12k.
9000x12000 is $108,000,000 upon this basic assumption this is a very huge dumping made, would this also create a huge plunge in bitcoin price? any math geek here?
Probably a bit? But not so much. It's like two to three drops in the big ocean. It'd make a ripple, but it shouldn't even make lasting effects to the market. He probably just wanted to reach the barrier of $100m before exiting, might have been his original goal back then when he bought that amount.