Let's say that winning ALL slots on a market dominant platform does have a value unto itself, and the total value is V. Then Google can consume the spots itself

Let's say that winning ALL slots on a market dominant platform does have a value unto itself, and the total value is V. Then Google can consume the spots itself, but it is giving up the ability to sell the package of all slots to someone else for V. So it still costs them V. If Google is the only one who can extract V (likely the case, since they are the ones consuming the spots), they are reaping the benefits of that fact, not of being able to sell to themselves.