I agree that the mere coincidence as reported (if it is valid) is not sufficient to tie a Keto type diet to A-fib.
I developed A-fib over the years and I would say if anything is related to developing a-fib it is living. I was a long distance runner when my heart murmur (the beginning of a-fib) was noticed by my doctor. But it became progressively worse as I aged.
After having fully developed chronic a-fib I started to use a low carb diet to control weight, The resuling weight loss led to a lowering of the diabetes marker A1C and a more defined heart rhythm. (Not out of a-fib, but more easily controlled with heart rate and blood pressure meds.)
I would say that reports like this should only result in closer cooperation between people with symptoms and their doctors.
As for a low carb diet, it seems to reduce the level of blood sugar. This is likely to have positive health effects.
Did lowering your a1c help your hemoglobin carry more oxygen?
Did you notice increased mental clarity?
I’m just going this route now and taking BHB to increase ketosis.