I am fairly sure that billionaires invest in Bitcoin and other crypto's, but the thing is that they prefer to remain in silence, out of the publicity.
It has always been like that ~ the most wealthy entities remain in silence when it comes to investments and gains, where others (e.g Kim Dot Com, Max Keiser) start being loud for no reason.
I can say that they must be investing in bitcoins. Because once person go at higher position they used to find out other ways of income having minimum government interference. It can be good option for such people. Right now normal people are also earning good ammount so may chances that billionaires are earning by it.