The study this is based on is here:

Background The influences of low carbohydrate diets in cardiovascular disease are controversial. Few studies have examined the relationship of carbohydrate intake and risk of incident atrial fibrillation (AF). We aimed to evaluate the association between carbohydrate intake and the risk of incident AF in the Atherosclerosis Risk in Communities (ARIC) study. Methods and Results We included 13,385 participants (age, 54.2±5.8 years, 45.1% male and 74.7% white) who completed a dietary questionnaire at baseline (1987-1989) in the ARIC study. The primary outcome was incident AF, which was identified by electrocardiogram performed during study exams, hospital discharge codes and death certificates. We used multivariable Cox’s hazard regression models to assess the association between carbohydrate intake and incident AF. Restricted cubic spline with 4 knots was used to express the dose-response association. We further explored the effects of specific food source (animal-based vs plant-based) used to replace carbohydrate intake in low carbohydrate intaking setting. During a median follow-up of 22.4 years, 1,808 cases (13.5%) of AF occurred. The hazard ratios for incident AF associated with a 1-SD (9.4%) increase in carbohydrate intake as a percentage of energy intake was 0.82 (95% CI, 0.72, 0.94), after adjustment for traditional AF risk factors and other diets factors. Results were similar when individuals were categorized by carbohydrate intake quartiles: the HR for incident AF comparing the 2nd, 3rd and 4th quartiles of carbohydrate intake as a percentage of energy to the 1st quartile were 0.79 (95% CI 0.68-0.92), 0.77 (95% CI 0.64-0.93) and 0.64 (95% CI 0.49-0.84) separately. Restricted cubic splines also showed a similar tendency of risk of incident AF in patients with carbohydrate intake in the range of less than 62% of daily total energy. No association was found between the type of protein or fat used to replace the carbohydrate and risk of incident AF. Conclusions Low carbohydrate diets were associated with increased risk of incident AF, regardless of the type of protein or fat used to replace the carbohydrate.

They used data from the ARIC study, where participants were given dietary questionnaires in 1987 - 89 and then followed up every 3 years, ending in 1996-98.

While I can't get access to the full studies, I can see the issues immediately:
1. Dietary questionnaires
2. long gaps in followup
3. Old data.
4. ARIC was not designed to study Afib or the effects of diet on cardiovascular outcomes. From their own website "ARIC is designed to investigate the causes of atherosclerosis and its clinical outomes, and variation in cardiovascular risk factors, medical care and disease by race, gender, location, and date"

So I'm guessing this is just another candidate for the circular filing cabinet.