It all depends on how liquid those assets are. "Paper" billionaire or millionaire typically refers to situations where someone is a millionaire but wouldn't be

It all depends on how liquid those assets are. "Paper" billionaire or millionaire typically refers to situations where someone is a millionaire but wouldn't be able to come up with a million dollars in funds in the short term (in less than a week, say). Paper ones might be able to take a haircut on their assets for additional liquidity, but that haircut might be enough to make them not a millionaire.