An exchange that publishes its own solvency has proved nothing: the figure and the thing it describes come from the same party. So this page asks two sources that cannot collude. The venue is asked what it owes. The chain is asked what is held. Your browser does the subtraction.
Each venue is audited separately, because they are separate systems with separate ledgers. This page covers openibank.com only — openibank.org has its own.
You should not have to trust this page either — it is served by the same company whose books you are checking. The verifiers are public and run standalone, against the chain, with no part of our infrastructure involved.