I'd have to at least partially disagree with both parts of that statement. The president can only do so much to influence the stock market. A lot of how it goes is due to the state of the economy that is inherited when they gain office. That said, the Trump tax cuts and dismantling of environmental and fiscal regulation probably did help the stock market (not to say I necessarily agree with those moves). However, I would definitely argue that much of his lying was in service of propping up the stock market, some of it likely successful. Downplaying the severity of the pandemic would be the latest and most obvious example, but there are many others.