If you want to work with this person, I think you would be nuts not to respond! It sounds like he/she is trying to recruit you for an RA position, which usually has funding and benefits. And if you don't respond (even if you pretend you never got the message) you're going to create the impression that you're unreliable and not very enthusiastic. I think that would be a lot worse than telling a theoretician that you are an experimentalist or whatever. Just my
!