That is their promise, and I've ready previously that they do have some audits that verified it (unlike Tether, which used to make a similar claim).
However, another important question is what actually happens to that dollar - are they actually able to wire you that dollar if you hand them back 1 USDC? For 3.3bn of those dollars, we know they are not currently able to, since those 3.3bn are tied up in SVB, which isn't itself able to wire them to anyone. Hopefully, the rest of the them are more available, but it remains to be seen.
However, another important question is what actually happens to that dollar - are they actually able to wire you that dollar if you hand them back 1 USDC? For 3.3bn of those dollars, we know they are not currently able to, since those 3.3bn are tied up in SVB, which isn't itself able to wire them to anyone. Hopefully, the rest of the them are more available, but it remains to be seen.