The wiki article doesn't explain, but the article it references does:

“What happened,” says Priscilla Oppenheimer, the Padres’ director of minor-league operations, “is that he had a virus around his heart. He’d just undergone a physical, too, but something like that can only be picked up on an ecocardiogram.”

I'm NAL, but with the presence of the virus, I'd think both workplace insurance and workers' comp would've been very combative over the claim, especially since it happened while he was only warming up which is far less stressful than actual gameplay. His family could (and maybe did) fight and so they are possibly getting some additional income from it, but I'm not sure how well the protections and such were almost 30 years ago.

Also, on top of the Padres still employing him, they apparently released him from having to repeat the bonus he got for the contract he couldn't fulfill, so that's pretty decent too.

Not necessarily. You can still suffer personal medical issues while at work. In order to be work-related you would need to prove that your job caused the heart issue. For someone like Demar Hamlin that’s pretty easy, but for many others it’s not so cut and dried.

This happens a lot in other industries too. I actually had an employee with suspected cardiac issues at work yesterday. They were taken offsite to a hospital. It’s been filed as a personal medical issue.

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most people who have heart attacks at work are covered forever via insurance? i don't think that's how it works, but maybe i'm wrong??