The pundit's claims are part of the speculation. If you speculated that the pundit's words would matter, you'd short or sell out grain, making the pundit's words matter. Supply and demand are only sometimes good measures of people actually using or producing these products (and when they fail bubbles occur), but are always good measures of the spending of both people actually using the product, and the investors buying it in wanting to sell it.

In any case, as the investors buy the product they eventually have to sell it, meaning over the long run demand and supply are good measures of "actual" usage and production. You know this because there's never a bubble that doesn't collapse and return to equilibrium.