T-Bills are independent from state of economy on account of having back up faith of US Treasury. In other words, T-Bills are risk free from default probability because the return of 5.5% will be realized irrespective of state of economy. US Treasury will have to redeem the T-Bills in any economic state.
However, the rate of 5.5% is composed of Risk Free Rate(2.5%) + Inflation premium(3%) and since there is uncertainty about level of inflation in the future, T-Bills are not purely risk free in terms of purchasing power. For Example if Inflation averaged to be 3.5% during