Do they hold assets other than their customer’s deposits? Are they even a bank?
I don't think they do any fractional reserve banking though (at least they didn't two decades ago), so there shouldn't be any liquidity risk. All dollars in deposits should be backed by real dollars.
As far as I know, paypal doesn't do fractional reserve banking (because it's not a bank and that would be illegal), it just collects a fee on the seller's side for each transaction, so everyone pulling out all of their money at once would probably not be that disastrous, supposedly they do have that money just sitting in an account. It would be disastrous to paypal as a business, and they would probably use fine print to keep that money, but out of greed, not out of lack of liquidity.
This is fine for many use cases, but totally different from the paypal experience.