Tricky question! If you are asking for the grammatical use of "as," it's a pronoun.
Check out definition #14 here: "14. a fact that: She did her JOB well, AS can be proved by the records." Here "as" is referring to "job."
In your example, "as" is referring to the quote.
This is my best answer. I have never known that "as" could be a pronoun until now. I wouldn't worry about how to use it -- as I can only say use it the same way you see it being used by others!