The article mentions the real core of the issue, but doesn’t make it clear that it is in fact the core: SLR (a banking regulation). SLR effectively requires banks to raise capital along with deposits. So if you flood the banking system with cash through fiscal policy, they can’t just accept all those deposits and leave them parked in their Fed reserve account. If they could, then the Fed wouldn’t need reverse repo—it could just pay interest on reserves to set a floor on interbank lending rates.