You think in the US the state controls the billionaires?
Also I'm pretty sure Cuba is still socialist even though billionaires exist outside of Cuba. Hell, some of those billionaires might somehow own shares in enterprises that exist in Cuba, which would mean there's workers in Cuba being exploited by capitalist billionaires. Does that make Cuba no longer socialist? Would it become not-socialist if the billionaires were Cuban instead (which would be preferable to being exploited by foreign imperialists, by the way)?