That figure was the market cap, the price at which a token was sold multiplied by the number of tokens on the market.

If a billion tokens are available, the market cap is $0.5B if you sell a single token for $0.5. That's what happened yesterday more or less. You won't be able sell all tokens for $0.5B, so saying they are worth $0.5B is misleading for such a volatile and young project.

The metric works much better (but not perfect) for established shares, financial products or contracts for physical objects.