For sure billionaires invest in btc. Why wouldn't they? They probably invest in a lot worse things. I imagine most billionaires, even though they have billions, would have noticed the amazing rise of bitcoins by now. These kinds of people have an eye for that sort of thing. Billionaires didn't become billionaires because they waited for the mainstream media tell them what to do like most of the sheeple do.
I think you're right. If billionaires are still looking for investment opportunities they would've certainly heard about bitcoin. Some might see the tremendous potential it has. Some won't.
I think that billionaires still invest even if they're already stinking rich. They would still find ways and places to invest especially since they have the resources to. They got rich in the first place because they have been diligent in finding the best investment and I'm pretty sure that they must have heard about Bitcoin in their search. They continue to become rich because they manage their finances and investments well. Investing on Bitcoin would be an addition to their numerous assets.