You’re right to point out this is an observational, retrospective study showing only a correlation. So there are a lot of caveats there. I think this raises some potential concerns though that should be looked into further. Ideally in a prospective and more controlled way to better isolate the effects of intermittent fasting.

There’s been many times in medical history where something that seems to make sense doesn’t pan out in the end or even causes harm. Studies like this are just step one, even the authors state they were expecting to show benefits and that this result was a complete surprise to them. We definitely need to be looking into this further to better quantify potential risks and benefits of intermittent fasting, especially with its popularity. It would be a shame if intermittent fasting did turn out to carry this risk though, there’s many people like you who have had success with it. It may be in the end that there are some populations it’s appropriate for and others it’s not. Needs more study for sure.