My theory for Fed fiscal stimulus: In 2008, the Fed started buying things that weren't treasuries (e.g. Mortgage Backed Securities (MBS)), and bulking its balance sheet up from ~800 Billion to ~4 Trillion. Some portion of that ~3 Trillion went to buying securities for which there was no other market at the going price. The difference between the market price (what a non-Fed buyer would pay) and what the Fed paid was stimulus to the firms selling the securities. They could turn around and spend that difference in to the economy.