Hello Inertia,
Just like another poster mentioned, I don't think it will matter a great deal that you asked the question. Odds are the professor or the GDA (GSA?) who called you will not even remember whose comment it was anyway. Also, they cannot blame a stressful applicant for asking such a question. Just relax and pretend that it never happened (unless they bring it up over the interview in which case you can just make a joke out of it.)
Good luck for your interview! I am sure it will be great.