The idea is that the exchange publishes a sum of their liabilities, and each individual user can check that their balance was uniquely included in the sum, cryptographically.
Unfortunately this does not prove in any way that the exchange has included all liabilities in the sum, it only proves your deposit is included. A very, very different thing.
Of course. But it lets every individual check this, which means if any individual's balance is not included, they can publish that. It is a vast improvement over the current state.