Authentic is a word we use often these days in education. From professional resources I've read (including the Sisters) and in my teaching, this is what I believe it to mean:
Authentic literature is material written for the joy of the story, not to fit a prescribed phonics "rule" or strategy. When published authors write stories, they certainly attend to their audience and the methods in writing stories (story elements, problem/solution, etc), so we can use these "authentic" texts to teach those strategies students need to develop an appreciation for and enjoyment of reading for pleasure. We are so lucky to be teaching in the era of publishers and authors who are developing to write both fiction and expository texts with all the story elements/expository text guidelines at various reading levels for all our readers.
Authentic writing is writing when students write about either true (authentic) events from their lives/experiences-- OR ideas from their imagination that follow story structure. Contrast that to always writing about a given prompt. Are there times for prompts? Certainly, but not as the "rule".
Worksheets can be helpful for practice--BUT, what we have to watch out for is that the "cuteness" factor doesn't take over what we're trying to teach. Also, we don't want to use worksheets that "drill and kill"--50 math problems when 10 would show if kids have understanding, or 20 words to look up in the dictionary when a class discussion of the words will give more understanding. `
I think, like all of our teaching, we need to consider the best way for our children to learn what we want to teach them. Thinking, talking and sharing--either orally or through writing--seem to be the most authentic 
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