Not correct. The Fed does not buy assets from investors. It buys Treasuries and MBS from Primary Dealers. The original seller of the Treasuries is the US Government. The original sellers of the MBS are typical the GSEs but my be investors in some cases. The first trade in the execution of monetary policy is the Fed's purchase of the paper from the Primary Dealers. Their business is to accumulate more inventory, mark it up, and distribute it. Investors are the customers of the pump and dump. They are the marks.
And when we say "investors" we're talking about the whales-- the institutions. Not the guys at the dollar slots, roulette wheels, and $5 blackjack tables.