I have not heard that report. It is possible, I suppose, but still not related to failure to wear a seat belt during landing. And, if true, the Heart Attack is due to the accident anyway.
Btw, the official name of a Heart Attack is Cardiac Arrest. Which just means the heart stopped. Yes, normally, death will cause the heart to stop.
It is far to soon to say the heart stopped due to a bad heart. Whomever reported that could not know.
Although this was obviously an accident and apparently the Pilots did a good job getting the jet on the ground, there are still people who want to defend Singapore Air and blame the passengers.