We are talking here about people that we DO know. There are a lot of crypto billionaires we don't know about. And we never will. Take Satoshi as an example. He is a crypto billionaire and we will never know who he is...
Privacy is one of the many big features of cryptocurrency that's why we should not expect these wealthy people to be sharing who are they are...though transaction trails are always viewable in the blockchain. Now, if one of these people, I don't have any obligation to the public though if I am selling my coins I would not have any choice but to declare it to the governmental tax agency and must pay the tax required.