He invested between $15-$20m in mid 2017.
With the price increase their holdings have increased drastically. If they were to cash out a significant amount then it would affect the price leaving the rest of their coins worth less.
The same goes with anyone holding large reserves looking to cash out. They sell small walls so they don't affect the price allowing the market to set the price and selling their coins in batches at the otherwise higher rates.
Still, I doubt OP's concern will ever affect the price. That is a guy who I am sure is not really bothered about cashing out anytime soon and even if he really wants to do, he would not just go out all at once. To him, the amount he has thrown in should be more like a trial and error kind of thing, and with some of these big guys really being a part of the community; you should even expect better days ahead for bitcoin.