Not true. I think Hive account@abit is the top investor in the top 19 witness list, and one of the top investors in all of Steem. He converted quite a lot of Steem Dollars (6 figures) into STEEM and most of it got powered up. Other witnesses such as Hive account@smooth and Hive account@blocktrades have powered up a lot of liquid STEEM that they could have otherwise sold on the market.

Witnesses have invested in Steem in other ways than money, too. There's generally a reason why someone gathers enough approval votes to be there.