Evolutionary success can be measured in many ways, and the sheer number (especially of individuals) of cnidarians today, in both freshwater and marine habitats, points to their success. "Highly evolved” is also ambiguous. Given that cnidarians have been around for more than half a billion years points to their probably having evolved to adapt to changes in the watery habitats in which they live, whether or not their basic body plan has been altered.
Work Step by Step
The key is to review the indicated earlier parts of the text in Ch. 25 and see that the terms in quotation marks in the text are ambiguous. From there you can build a “defense” of the status of cnidarians.