I don’t know any details about this case but if he sold his bitcoin after he renounced his citizenship, he wouldn’t have to pay taxes in the US, would he? If he sold before then of course he would have to pay in the US under the law
He didn't sell.
It a "constructive" (legalese for imaginary) sale that happens by law when one renounces citizenship, triggering capital gains taxes on the value of assets held at that time.
It's a wealth tax designed to deter citizens of means from renouncing citizenship (the US taxes citizens earning abroad).