This strikes me as either your cardiologist being wrong or you misinterpreting what they said.

Can you damage your heart doing endurance events? Most certainly. I’ve seen at least two cardiac arrests during events where the person did not survive.

Does everyone who does an event suffer heart damage? No.

There have been quite a number of studies mentioned in ultramarathon journals that contradict your claim.

Additionally, I have run so many marathons and ultramarathons I’ve lost count. I just recently had my own extensive (and expensive, despite insurance) cardiology work where they observed no such damage. I asked them whether endurance sports cause heart damage, because I’m old and thinking of starting up again and the summary of their answer was “as long as you’re training properly, you’re good”.

That said, certainly these events put short term (~weeks/~months, depending) stress on the heart.

The article mentions new medical technology and techniques as increasing survivability but I also wonder if better training and awareness is also a key. When I started running ultras particularly, it was just a bunch of nutcases. But now you have “hobbyists” like soccer moms running them (and I’m thrilled about that) and doing well at them because there are sets of extremely good training knowledge out there.