Advice that says "don't change someone else's post, create your own" is often meant to prevent people from hijacking questions or changing the meaning of answers. It happens sometimes that a user will edit a post to add something like:
I have the same question, but in my case the frobniz causes a "Frobnitz creation error"
as soon as it's called. Can anyone suggest a solution?
That sort of thing takes the question in a new direction, confuses the issue, and possibly invalidates existing answers. In short, it changes the intent behind the post. On the other hand, it's quite reasonable to edit a question to add information relevant to the OP's intent. For example, someone might run the code included in the question and add the output to the question to help others to better understand the issue.
It's the same with answers: edits that change the author's intended meaning should be avoided, but edits that expand on or otherwise improve the answer are usually helpful. In the case you cite, the editor added a fact that wasn't available when the answer was first written.