What does your realtor say?
All properites are basically "as is" with no implied warranty in my opinion.
I think what they are telling you is that it is an old house and they don't want to do any repairs.
I have often negotiated around this "as is" information anyway.
Let's say they tell you it is "as is" and you agree, then you do your home inspection and find out it needs a new roof and you cannot get insurance without the new roof. That might be a case then where the seller is then willing to put on a new roof to get the house sold. If you have an option period you can always try to renegotiate or back out during the option period.
I don't remember ever seeing a seperate "as is" addendum other than for foreclosures. Is it a promulgated form or have they drawn it up themselves. Please tell me you are using TREC contracts and not something from an office supply store or off the internet.