I never said no to availability. Clustering definitely increases both the availability and reliablity of the Application.
You are reading too much into things. Just because someone writes an article that says "Clustering good, better Availability", doesn't mean that this is the only benefit. It just happens to be the focus of the article.
Don't forget that there are two types of Scalability: Vertical and Horizontal. Clustering is a form of Horizontal Scalability. Most
J2EE Application Servers support both.
Regardless, you are getting into too much detail. The exam itself is very high level when it deals with these things. Questions are very straightforward.