Even if a bunch of people used cheap money to buy TSLA, that means a bunch of other people now have lots of money they made selling TSLA. So someone, somewhere

Even if a bunch of people used cheap money to buy TSLA, that means a bunch of other people now have lots of money they made selling TSLA. So someone, somewhere, is sitting on cash, which is deflationary and not what you want in a weak economy. The cure for that problem is cheap money plus expected inflation (cheap money so people can get cash, expected inflation so they go spend it). The Fed can make money cheap, but it can't force people to expect inflation. It can do stuff to encourage that expectation, though, and yeah it's doing just that.