Probably not dollars, but some kind of low-risk assets like USD bonds? In that case you also can't just withdraw dollars, you have to sell the bonds - if they have to convert large amount like hundreds of millions, the spread on those bonds might get larger.

Also even though it were just dollars in a regular bank account, when the amounts go large enough, the bank might have liquidity issues of their own.

What would be the biggest issue is the compliance. As rumours are circulating that Binance might be getting some kind of charges, the banks and other institutions might just stop doing business with them because that would increase their risk a lot.