Interesting. Seems he was born from jewish parents, but in 1973. Back then, circumcsiion for religious reasons may not have been allowed. Lost of jewish malkes in the Soviet Union were jewish by having their mother's "jewish" certificate since they hand't undergone circumcision at birth.
But Brin moved to the USA at age 6 with his parents. Not sure if they "escaped" or if they bought their way out of the Soviet Union. Either way, it is likely, but not sure, that he got the snip when he got to the USA.
There might be an interesting story behind this.