If your calculations are correct, then yes, it’s theoretically possible for billionaires to buy up 90% of the total bitcoin in circulation. However, in practice, this would be extremely difficult. Not many billionaires actually believe in bitcoin, as most prefer to invest in physical assets.
Even if they collectively decided to buy, the price of bitcoin would skyrocket due to the surge in demand, making it unreasonably expensive even for them... on top of that, many current holders would likely refuse to sell, further limiting the supply and making it even harder for billionaires to accumulate such a large portion of bitcoin.