Yes, I do think that billionaires invest in bitcoin. They are people with smart business capabilities and they know that bitcoin is a growing cryptocurrency and that it would bring huge profits to them.
Even in the early stage of bitcoin they already invested for sure because that is how they roll in this kind of industry and most of related things about businesses or getting profits. If they know that there is a new opportunity, they will work hard on studying it and if they found that it is really worth it to invest then they will invest for sure.