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 believe in this one. I think they are investing in bitcoin but they are not public stating about it. Wealth diversification and wealth accumulation will always be their main objective. So they will always be open in new ventures and ideas for their finances. I'm sure some of them knows about bitcoin. Most probably, they are into trading and mining since they have enough cash and resources.