It was a great decision for them, without any doubt. But I can hardly imagine that the crypto market got advantages from it. After such a solid amount of ETH was sold, the market crashed and ETH fell to 80 bucks per coin.
Firstly, this amount was sold in January 2018 and this did not seem to had an effect on the price being $80 after 1 year. Also, I do not think he used exchanges to sell ETH, because it was enough to influence the price.
But there is another situation. Lee, founder of Ltc, made a similar statement. It's the end of the bull right in front of us!
I do not think it will have the same result. IIRC, while Lee sold all his LTCs, Buterin only sold some. And Buterin made a statement about it almost two years later.