Does it make sense to say that, in a way, US Treasury bonds are more liquid than USD? I think this because the value of all the bonds in circulation is way greater than all the dollars in circulation, so the largest purchase that bonds could facilitate is much greater than the largest purchase USD could facilitate. For example if you wanted to buy $5 trillion worth of oil, there's not enough USD in existence to pay the bill, but there is enough bonds to do the deal.
edit: To clarify, I'm not talking about physical cash I'm talking about M1. I'm also only talking about liquidity for very large transactions with the idea that, for example, $1 is more liquid than $1 trillion in the sense that you can "sell" it (for oil or other goods) at less of a discount. The key idea/question is that liquidity of an asset (USD) is relative to the quantity you want to sell. Even if the oil purchase, in the example I gave, was less than the value of all USD in circulation, buying then selling huge amounts of it would come at a cost that's greater than if treasury bonds were used. And buying or selling huge amounts would also effect it's intrinsic value just like any financial asset.