Yes, unfortunately this is one of those situations where English is either wonderfully flexible or a terrible mess...depending on your point of view. Some people will argue that all the nouns that end in -ism should then be -ist(noun) for a person who follows that belief/practice and -istic(adjective) to describe a thing with that quality. But there are lots of examples (socialist and communist are two of the most common examples) of adjectives that end -ist, and many examples of words (Biologist, Physicist for example) where we never say -istic as an adjective