That is my impression as well. An ultra marathon is not a walk around the block even if they both commonly involve moving your legs.

Moving your legs might not be for everyone!

Yes but walking is rarely the cause that issue.

'Walking is not for everyone' and 'Exercise is not for everyone' is technically true since it is going to do more harm than good for a small subsection of society.

It is even more true that some people should not run marathons or ultra marathons, but it would be odd to notice that then write an article based on that evidence that 'walking is not for everyone' rather than 'marathons are not for every one'. And when I have read of serious consequences of meditation it is from doing the equivalent, or more than, an ultra marathon, but then the conclusion is the equivalent of 'walking is not for everyone'.

I'm always amazed at how many people are experiencing meditation for the first time who do this. It must be like suddenly being arrested and stuck in solitary among lifers.

On a related note, I've enjoyed my time so much staying at temples that the idea of being stuck in solitary for a long stretch is like a weird ambition! If you see my name in the news for a crime you'll know why…

My anecdotal experience is that this is a personality type. "Some is good, more is better" is just not a valid way to approach things, neither in quantity nor in intensity.