These were done for strength and lightness I believe. Cored hull is fine so long as the core is dry and hull not delaminated. I believe a survey with boat out of the water could easily detect these problems.
J boats tend to have areas of hull not cored near centerline for thru hulls, etc... which makes water intrusion into core less of an issue.
J boats usually have cored deck..Deck and coachroof have a LOT of holes for fittings, etc... which are great paths for water to follow to soak into core. While deck recoring is not nearly the work of hull recoring and not nearly the issue it is where I suspect you would have more problems in this 20 year old boat.
If you like the boat make the offer subject to survey and have the surveyor check for wet deck and hull. If these areas check out as fine then you will have a great boat that sails very well.