Important distinction: wrist-based HRV relies on photoplethysmography (PPG), which estimates inter-beat intervals from blood flow rather than directly measuring cardiac electrical activity like ECG. That inevitably introduces more noise and wider margins of error, particularly during motion, and Garmin devices are primarily designed as activity wearables.
However, under low-movement conditions (especially overnight), PPG-based HRV can still provide meaningful longitudinal trends. For most users, trend tracking is arguably more relevant than absolute ECG-level precision.
It's also worth noting that the study appears to have tested an older Vivosmart 4 using Garmin's Elevate Gen 3 sensor. Newer Garmin devices use later-generation sensors and updated algorithms, which may perform differently.