I don't think every billionaires are investing in btc, even if they do not know about this btc, but as the btc grows, and has increased significantly in the past few months, I'm sure sooner or later these billionaires will at least hear about btc this, and did not rule out they will invest into it
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.