Institutional investors have so many purchase options that they do not need to use backdoors.
This seems to me one of the many fake news that would claim to influence the market
They're the best options available. Why would anyone with deep pockets go to an exchange and wreak havoc with the order book? Not only would your pricing go haywire, you'd get preyed upon by all the little shits lurking.
OTC is by far the most sensible route for many.