Unifiers are people whose driving passion is to find general principles which will explain everything. They are happy if they can leave the universe looking a little simpler than they found it.
Diversifiers are people whose passion is to explore details. They are in love with the heterogeneity of nature … They are happy if they leave the universe a little more complicated than they found it.
Presumably these categories correspond to what Freeman elsewhere calls birds and frogs, or what others call hedgehogs and foxes. I imagine everyone takes pleasure in both unification and diversification, though in different proportions. Some are closer to one end of the spectrum than the other.
The scientific heroes presented to children are nearly always unifiers like Newton or Einstein . You don’t see as many books celebrating, for example, a biologist who discovered that what was thought to be one species is really 37 different species. This creates an unrealistic picture of science since not many people discover grand unifying principles, though more find unifying principles on a small scale. I imagine many are discouraged from a career in science because they believe they have to be a unifier / bird / hedgehog, when in fact there are more openings for a diversifier / frog / fox.
Dyson may be taking a subtle swipe at unifiers by saying they want to leave the world looking a little simpler than they found it. There may be an unspoken accusation that unifiers create the illusion of unity by papering over diversity. True and significant unifying theories like general relativity are hard to come by. It’s much easier to come up with unifying theories that are incomplete or trivial.
 Or at least scientists best known for their unifying work. Newton, for example, wasn’t entirely a unifier, but he’s best known for discovering unifying principles of gravity and motion.