A main feature of a betting market is that it gives you a way to turn insider knowledge into money. If you can place a bet based on info you know is good that you know other bettors don't have, you can get favorable expected return.

Thus, avid bettors have incentives to get the best information possible, while people who happen to encounter unique information have incentives to become bettors. If we assume people are placing bets rationally, we can assume that bets that move the line are in response to some good info or insight. Thus, the betting market reflects all available information everywhere.

I guess my point is, it's not necessarily that big bettors are good at getting info. It's that any info that happens to fall into the hands of a potential bettor will incentivize them to become a big bettor. I think in reality, successful bettors are good at getting info and getting an edge. The ways to do this are always changing and are their secret sauce. Also, even if only a small number of bettors have the info, they will act and that will move the line. I guess my point is that the market is a wide net and any participant who knows anything has incentive to place bets accordingly.