I agree. When you look at the baseline differences between the groups, though not of statistical significance (p=0.09) you also see that the "recovery" was larger in the control group, which had the lower baseline value. At the end of the experiment, both groups were closer to each other than at the start. That makes me suspect that what we see is a regression- to-the-mean effect, that leaves the difference at no statistical significance at all. But anyway, the RHI is, in my eyes, not the best indicator of judging arterial health. One reason is that it only reflects arterial response at the limbs. It also is a mixture of endothelial-dependent and non-dependent response. The ability to dilate also depends on the baseline arterial lumen itself.

But anyway, RHI and PWV measure distinct functional parameters. And yes, you are correct to say that PWV is a measure of stiffness of central arteries, whereas RHI is a measure of dilation of the peripheral arteries. Both can help to identify the functional status of arterial health. I used PWV in my quick self-experiment, because I work with PWV and do not have the Endopat system available.