"(Subj x)(verb) as y did" describes a more common pattern for the use of "as" and "did" in sentences. This pattern is used to directly compare the actions of two things. As an example, you collude this grammar pattern to say something like "He ran quickly, as his sister did," or "The corn stalk grew differently than the tree did."
Mike presents that first line in the notes to make it clear that the actual sentence is NOT following the common (subject x)(verb)(as y did) pattern, but is instead following a different pattern and communicating a different kind of idea.
You've brought up an interesting question. Choice (C) is definitely the most correct answer based on the parallel structure issues discussed in the explanation. After re-reading the sentence several times, "return in" seems more appropriate to me (to denote the period in which the comet returned). That said, I have not seen any other comments here or in other forums about the incorrect use of "return of" in this context.
Remember, for SC questions, you're looking for the most correct answer. So, even though "return in" could be seen as more correct than "return of", (C) is the most correct choice based on the arguments of parallelism.
3 Explanations