A partisan bet makes the opposite position slightly more profitable, attracting bets which equilibrate the market back at fair prices (i.e. realistic probabilities)
A whale just means the market gets more mis-priced, creating greater incentive for correction, which typically occurs.
Let's for a moment say you're right, and prediction markets do have a flaw (that if someone wants to spend a lot of money making an outcome look more/less likely), that's an imperfection vs the many biases in other information sources.
tl;dr prediction markets financially penalise providing misleading information, whereas traditional media profit from misleading information
