Thanks for pointing this out, Mark. Here is why I don't trust this device and similar ones like the Oura ring:

The device records a PPG signal at the arteriolar/capillary level of the vasculature (finger). The recorded wave is then mathematically deconstructed to derive the PWV on the upstream arterial level. While it is conceivably possible to do this for a given individual after calibrating the devices against an actual PWV measurement, I doubt very much that such calibration data are applicable across user populations.

The PPG signal is very sensitive to personal and environmental disturbances and idiosyncrasies. That’s why I doubt its reliability. It has also become apparent in cross-validation studies that the reliability and accuracy drops along the distance between the heart and the signal acquisition point. That is, armbands are more reliable than wristbands which are more reliable than finger sensors.

What these PPG signals can be used for with acceptable reliability is to measure heart rate and possibly heart rate variability (HRV).

Even if these problems can be overcome, you’d only be able to derive the PWV of the closest upstream vascular path (brachial-radial) but not the central PWV. brachial-radial PWV is a poor indicator of vascular health since the muscular arteries don't change much over the course of a lifetime. It's the central, large vessels (aorta) that are far more sensitive to aging and its acceleration or deceleration by lifestyle choices.

All this conspires against the usability of iheart and the ring for the purpose of measuring PWV reliably.. I wouldn't trust those devices.

As an aside, the iheart claims superior signal resolution to other devices. It records the PPG signal at 200 Hz. The medical device that I had the opportunity to help develop had 5 times the resolution (1 kHz).