Not entirely true
Basically - the best way to use HRV is under the same conditions every day.
Garmin only does it when you’re sleeping (at least by default). Basically - lying still, not moving, pulse is at your resting heart rate. It doesn’t do it during the day because to my understanding, the data isn’t reliable that way.
The other way to do it is first thing when you wake up, no coffee or any food. Sitting or lying down and not moving so your RHR can return to baseline. I used an app by Morpheus and a forearm sensor, but didn’t feel it gave me better data than the Garmin and it’s a pain to do it every morning.
Garmin works for me because it takes a bunch of readings throughout the night, you can see the graph / plot, and in my experience it’s tracked well with recovery, if I’ve had a hard workout, if I had a drink or two, or if I was getting sick