the industrial complex surrounding the Stalingrad Tractor Factory produced forty percent of all T-34s before being cut off by the Battle of Stalingrad, in autumn 1942. T-34 - Facts, Information, and Encyclopedia Reference article If so it adds considerable weight to the value of capturing Stalingrad.
I think what the article is pointing out is that, while Stalingrad was threatened, T-34 production was being moved to other sites. The 40% figure seems to be while the other sites were moving and going into production. See the following article T-34 Medium Tank Production