This is somewhat true but there are a lot of muscles that aren't getting worked maximally. Further, if you limit a motion to a single plane, it will still recruit fewer muscle fibers in the main muscle group you're working. Smith machines limit the potential directions the weight can go in considerably (only letting it go up and down), BB press allows more freedom thus is more challenging (now the weight can go up, down, forward, and backward), and DB press allows even more freedom (now not just forward and backward, but also side to side). The more potential directions the weight can go in, the more likely it will be that you're recruiting as many motor units (and thus muscle fibers) as possible.