There's two forms of manipulation mentioned. One is changing the market to influence public perception, that does become harder as the market grows in size.

The other is accepting the bribe, sorry, taking the other side of the bet, and making something happen. That only becomes worse with scale. When you're in the position to accept a million dollar payout to cause the press conference to only last 64 minutes, or to invade a foreign country, suddenly you have a million new reasons to do so.

On any prediction market where a reasonably small group of humans decide the outcome, and there's enough money to matter, "betting no" is better understood as offering a fee to make it happen, conditioned on damages should someone accept your offer and fail to do so. "Betting yes" is better understood as agreeing to facilitate the outcome - or assisting in the price discovery mechanism that says facilitators are over charging.