Really? It's fairly common advice to store an emergency fund in something akin to a savings account (or at least something with FDIC backing). Either way, it seems bizarre for a high net worth individual not to have at least $500 in something extremely liquid like a checking/savings account.
Either way, it's fair to assume it means liquid cash, since the question is basically "could you afford an unexpected $500 expense?".
If you want to take that study to task, I think the far more interesting statistic is the 20% who would put it on their credit card: While those could be people who are just going into debt to afford the bill, it could also be people who just pay their card off in full every month and want whatever rewards they might be entitled to from using the credit card.