The crux of that letter is the "need to recognize that meaningful patterns of genetic and biological variation exist in our species that are not racial." This is true. However, this does not mean that there aren't also meaningful patterns of genetic and biological variation that
do stratify according to ancestry (not race!). The letter tries to handwave this away, claiming that "[f]or several decades billions of dollars have been spent trying to find such differences. The result has been a preponderance of negative findings despite intrepid efforts to collect DNA data on millions of individuals in the hope of finding even the tiniest signals of difference." This is simply not true, as studies like the subject of this discussion demonstrate.
The letter also states that "[t]he public should not cede the power to define race to scientists who themselves are not trained to understand the social contexts that shape the formation of this fraught category." Also true! This is exactly why Reich explicitly avoids discussing "races" but rather populations and ancestries, which are rigorously defined strictly in terms of genetics. With respect to population structures and ancestry, Reich is indeed an expert.
I'll add that very few of the signatories of that letter have any experience, let alone expertise in genetics. Here are the first few:
Jonathan Kahn, James E. Kelley Professor of Law, Mitchell Hamline School of Law
Alondra Nelson, Professor of Sociology and Gender Studies, Columbia University; President, Social Science Research Council
Joseph L. Graves Jr., Associate Dean for Research & Professor of Biological Sciences, Fellow of the American Association for the Advancement of Science, Section G: Biological Sciences, Joint School of Nanoscience & Nanoengineering, North Carolina A&T State University, UNC Greensboro
Sarah Abel, Postdoc, Department of Anthropology, University of Iceland
Ruha Benjamin, Associate Professor, Department of African American Studies, Princeton University
Sarah Blacker, Postdoctoral Research Fellow, Max Planck Institute for the History of Science, Berlin
Catherine Bliss, Associate Professor, Social and Behavioral Sciences, UC San Francisco
Out of the 67 signatories, I counted approximately 5 who might have sufficient genetics expertise to offer a meaningful scientific counterpoint to Reich's work (this is being charitable, as I included titles like "Professor of Biological Sciences," which is no guarantee.) The rest were in fields like anthropology, sociology, law, and history.