I think it is not a myth. If a group of people have a large number of tokens or coins, they can be easier to manipulate the market. Whales have their group or community, they may work together, they are very possible to make a change in a market or exchanges. I have seen many sources and discussion that explained or investigated about the power of whales. The conclusions are almost the same, they stated whales may be the main factor that make a big change in crypto market.