Higher cardiovascular fitness is associated with a lower resting heart rate and higher heart rate variability.

Study: Differential Adaptations in Resting Heart Rate Following Moderate-Intensity Aerobic and Sport-Specific Skill Training in University Athletes

People who do regular aerobic exercise, like running or cycling, end up with slower resting heart rates because their hearts become stronger and more efficient. This is a sign of better heart health, which usually means their heart also shows more natural variation in beats — even if this wasn't measured directly.

Study: Improving cardiovascular autonomic function in postmenopausal women with hypertension: a pilot study of supervised versus home-based aerobic exercise.

People who did regular aerobic exercise got fitter and their heart rhythms became more stable and healthy, which means their hearts were working better at rest — exactly what the claim says.

Study: Association between heart rate variability and cardiorespiratory fitness in individuals with type 2 diabetes mellitus

People who are more physically fit tend to have slower, steadier heartbeats at rest and more variation in their heart rhythm — this study found that in people with diabetes, better fitness directly linked to these healthier heart patterns.

Study: Description of age- and sex-specific reference values for cardiorespiratory fitness in healthy ethnic Kazakh adults: a cross-sectional study

People who are more fit have stronger hearts that don’t need to beat as fast when they’re resting, and their hearts respond better to changes — this study shows fitter people have better heart performance during exercise, which means the same is likely true when they’re at rest.