I think billionaires have invested in BTC, specially those who are rich because they own a tech based company like Bill Gates, who own microsoft. Who knows maybe the owner of Samsung and Apple might have invested into bitcoin but they don't tell others about it. It is also possible that Mark Zuckerberg invested on Bitcoin, if they do I hope they will consider to accept bitcoin in the future.
I don't know about Zuckerberg, but Winklevoss Twins, known for their legal battle with him, earned $11 million from their investments in Bitcoin.
But you are right, we can't know for sure whether billionaires invest in bitcoin and how much if they do.