It is a word, but not a common one.
English is not defined by dictionaries (but good work on consulting one). Instead dictionaries react to how speakers of English use the language.
Many adjectives can be formed by adding "ic" to the end of nouns. But this pattern is not completely productive. So we do have "Idealist" -> "Idealistic". But we tend not to use "Buddhist" -> *"Buddhistic". This might be because the noun can be used attributively "A Buddhist sculpture" which means that there is no great need for the adjective.
However, it doesn't mean that "Buddhistic" isn't a word. A word's nature is not determined by its form, (which sounds like a very Buddhistic saying). If enough people use the word, then the dictionaries will pick it up.
Now "Perfectionist" can also be used attributively so there is less need for a separate adjective, but nevertheless some people are forming "perfectionistic". For example Forbes.com writes "Thousands of women today are exhausting themselves through their perfectionistic overfunctioning." And dictionaries are starting to pick up it.
The existence of a German word is not a good guide to whether an English word exists. In particular, many English nouns can be used as adjectives or even verbs without adding a suffix.