There are some bets on prediction markets where the future is either already known or in the control of people who may be participating in the market. For example, when people bet on how long the next presidential briefing will be, it doesn't take a prophet to predict this, anyone who organizes said briefing can control it (at least with a very high probability).

So, the question becomes "what is the preponderence of such bets" and "how many people with control or knowledge of bet outcomes actually participate in the market" - not "can some people see the future of any bet better than others".