Newton and Darwin for the reasons you cite—and if two arrived at the same breakthrough at almost the same time, does one dare imagine that a dozen or more others weren't damn close, despite the fraction of a percent of the global population had, or has, the upbringing, inclination, and freedom to even approach the problem in the first place? Hell, does one even dare imagine that a thousand nobodies hadn't had the insight that would have led them straight to that breakthrough much sooner if only they'd had the background and environment to recognize it as significant, follow through to prove it, and gain publicity for their discovery? Surely not.
Aristotle, well, shit, everyone whose independent work survives from antiquity looks like a lone genius, but that's obviously, in part, because so little survives and because attribution is so sketchy. With Aristotle in particular it's not even clear how much was his work is his, directly, and how much was the work of his "school", developed over years or decades by many people, if we're talking about particular insights or inventions, and not sheer overall-importance of a person.
I know less about the environment around Einstein's breakthroughs but given the rest I'd be surprised if the author actually managed to pick a good example, for his purposes.