That's how it really goes, the law of supply and demand is the principle that's being followed by most buys and sells in the market.
So applying the same logic would mean, a coin/token holder that can influence the price could infinitely make profit buy dumping and buying again?
Yes, it's that's how really it is. And that's why there are famous folks that are also intervening in the market and giving their thoughts even if they're not experts of economics.
It's because of the influence and reach that they can do in the market and many of their followers would follow what they would say.