they are on topic, it's directly related to what happened, people think professional footballers = fittest people on the planet, some of them are yes but not all of them, probably fitter than armchair fans who sit at home all day and drink lager eating pies, but compare them to other athletes and they are nowhere near the pinnacle of fitness.
people also think fitness = they cannot die young, they can, recently a bodybuilder nicknamed "zyzz" died in his early 20's yet just look at his physique (google him), this was due to a heart defect.
they probably did happen, just weren't publicised as much as they are now, you can be extremely fit, but no amount of fitness is going to stop a defect in the bodies most important part from killing you.
phil o'donnell died a while back too, from a heart attack iirc whilst on the pitch.
this isn't a special case, it has happened before, extremely sad, but things like this do happen in life.
i hope he makes a full recovery and if any defect is found, their is hope of fixing or helping it so he can play again. but people with heart problems shouldn't really put them under extra stress regularly for obvious reasons, but their are exercises they can do to help them and sometimes even cure problems, such as high blood pressure, etc.