This is one of the classic problems of talking about HA: making it clear which "failure" or "availability" you mean at any given moment. In the particular quote

This is one of the classic problems of talking about HA: making it clear which "failure" or "availability" you mean at any given moment. In the particular quoted sentence, "failure" refers to component failure even though the "availability" in HA means system availability. The goal of HA design is to make a system that becomes more available as components are added instead of less (as you seem to expect). Redundancy, internal monitoring, and failover are the tools of the trade. What you should end up with is a system more like two chains holding up an object instead of one chain twice as long.