Opinion – The Economy Does Much Better Under Democrats. Why?

That's not entirely true, but, yes the Congress (partiularly, Republicans using control of either at least one House of the Congress and/or the Presidency, which has a veto over Congressional legislation) has largely neglected fiscal stimulus in the last couple decades, leaving only monetary stimulus by the Fed (not the Chair alone.)

But the history the article refers to is for the past 100 years, not just since the late 1990s.

> it doesn't matter who is elected

Aside from the fact that the Federal Reserve Board of Governors, including the Chair, who is not a sole dictator over the Board, are appointed by the President and confirmed by the Senate, who is elected matters because who is elected determines whether or not Congress neglects fiscal stimulus. “Who is elected” is why the trend you point to since the late 1990s is a thing: elect people who have a propaganda interest in people not seeing government as capable of stimulating the economy to positions to control government decision-making, and, surprise-surprise, government isn't as likely to effectively stimulate the economy.

The American Reinvestment and Recovery Act of 2009 was a thing, of perhaps too small and not optimally targeted and not followed up on. The CARES Act, and the secobd pandemic relief bill, were things (definitely too small and terribly targeted, but it happened.) It seems quite probable—because of who was elected—that a larger and differently-targeted third pandemic relief bill will likely soon be a thing.