> > 97% of these MBS were 10+ year duration, with a weighted average yield of 1.56%.

But why didn't they just hold money at the Fed given that they are a bank and they can?

It's literally splitting hairs between what the Fed Fund Rate is and what they got on their MBS.

Explainer post says end of 2021 they made that trade, in March the Fed raised the Fed Fund Rate to 0.20%, and by April it was 0.77%.

Had they waited just 5 months they'd have got a better Risk adjusted return by just keeping the money at the Fed