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.
If you have one node with the probability of x% that it fails within a year and you add a second one with the same probability to fail the total pobability that one of the two nodes fail within a year goes up.
* the more complex the configuration required to provide high availability, the more likely something will go wrong and reduce availability
* the more nodes you have, the more likely one of them will fail. Whether that reduces availability typically depends on the failure mode and control layer.
For an example of the latter, with the distributed database Riak (and I imagine others as well) if a server is starting to fail (corrupt disk, bad network card, etc) you’re in a much better place if it stops working entirely. A gradual node failure is much much worse.
I may be misreading your comment, but how is this true?