I will add to this a second punch line: there are other ways of simulating Model 2. One way, if you know what all the possible confounders are, is to measure and adjust for them. However, randomization does have one great advantage: it severs every incoming link to the randomized variable, including the ones we don’t know about or cannot measure (e.g., “Other” factors in Figures 4.4 to 4.6).
