That is quite a story and unraveling liability could be a difficult matter. I would encourage you to treat this like a car accident and not discuss anything but what you know as the facts with the new owner. Do not admit liability nor infer liability by agreeing to help without strict limits on the extent to which you are willing to help. Your statement that "we had no clue this would be an ongoing problem" gives credibility to the theory that it is in fact the same problem. While the symtoms are the same, the problem is different. You experienced a problem resulting from a broken pipe, while the new owners are experiencing high groundwater from precipitation.
I have been involved as an expert in similar situations. The most likely path would be for the buyers to attempt some form of arbitration rather than an outright suit. There are probably arbitration proceedings specified in your sales contract.
Turn this over to your realtor and a real estate attorney now before you speak to these people again. This is not the time to be nice or try to be understanding or feel guilty. You need to remember "We wrote on the disclosure form that the den had flooded due to a leaky sprinkler system which had since been removed". Period, end of discussion. No more explanation or interpretation needed. If you need to say ANYTHING more, don't.
There are ways to defend yourself from liability and everything you say, and do, or write, or share in terms of dollars, compromises that defense.