Vygotsky’s Ph.D. was in psychology, from a psychology institute, and he worked professionally as a PI in experimental developmental psychology. Indeed, one of the themes of his work was to try to find a scientific basis for the study of cognition. But nobody is interested enough in studies of Russian children from the 1920s to track down the tech reports. People today read his essays summarizing his theories (like the ones in Mind and Society) so they can understand the connection to later work. The theories were also influenced by philosophy because his undergraduate training was in the humanities, but that doesn’t make his work “just, like, words”.