I didn't find anything in the article that claimed he hadn't known he'd had two previous heart attacks. The article said he had two stents implanted, which I'm assuming, shows he had some type of obstruction problem.
Living in the Houston area, I am exposed to a lot more news stories about Enron and Ken Lay than the average bear.
It wasn't until the autopsy that there was any evidence that he'd suffered even one previous heart attack.