Maybe this would be better written as: "This is all that is needed for consensus in systems where one entity exercises control over all portions of the distributed system."

When Amazon builds a distributed system where they own all the pieces, that's one problem.

When a group of independent actors are trying to build a distributed system to agree on something, when anyone is free to join or depart at any time, and no actors may be treated as privileged or trusted, the problem is just a little bit harder.