Twitter was publicly traded at the time and Dorsey owned approximately 2-3%. The shareholders overwhelmingly voted in favor of the sale, effectively giving Dorsey no choice. He, in fact, didn't even sell, he instead rolled his shares over into the new company, which means he didn't take the payout implied and instead ate the decrease in value that has occurred since then.
I'm all for criticizing billionaires for the things they do wrong, but there are enough of those things that we don't need to invent fault where there is none! It's actually counterproductive to do so because it distracts from your legitimate points.