I think it's reasonable to be suspicious of overzealous use of inadequate statistics, but the article quite clearly stated that Black is not making unwarranted

I think it's reasonable to be suspicious of overzealous use of inadequate statistics, but the article quite clearly stated that Black is not making unwarranted claims about the accuracy. You're making a valid counterargument to a claim that was not made. I don't know how static vein patterns are, but in the particular case of varicose veins, my understanding is that they're existing veins that start to bulge outward. My guess would be that capillaries are pretty fungible, but the larger you get, the slower they'd change. For the best statistics, you'd probably want to incorporate the amount of time passing between two images, and model how much change is possible/reasonable. But there's plenty of signal to be had if you're careful to not overstate it.