The methodology section explains why this benchmark is a valid measurement of servlet engines. However, this benchmark is indeed valid for several reasons. First, it primarily measures overhead, the best possible performance for a JSP engine. Second, except for the overhead, JSP applications have identical performance. Third, the benchmark is not cacheable.
Overhead measurements are always valid and often the most useful measurement.
It tells you the best you can possibly do, i.e. the speed limit. Because it doesn't exercise any functionality except the overhead, it's impossible to defeat the measurement. The bulk of a servlet's work is identical no matter which JSP engine is chosen. The Java code runs at the same speed and the database drivers are identical. This benchmark would be less effective in comparing technologies with different languages and drivers.