As far as I know, Stalin or the general Bolshevisk government made the border in the 1930s, allegedly along ethnical lines, which don't coincide with what Stalin did in reality...
Your theory is very good... for insane asylum or Russian Studies professor of some American Uni. In fact, great deal of effort DID go to drawing of those borders. Thing with the Northern border of Kazakhstan, however, is, Kazahstan consists of Southern part (traditional tribal lands of Kazakh nomads) and Northern part (which used to be kinds of "no-man land" no tribe had substantial claim on before Russia annexed those lands). Stalin attached those Northern (mainly Russian-populated) lands to Kazakhstan due to many reasons, but none of those reasons work in Alternative realities.As far as I know, Stalin or the general Bolshevisk government made the border in the 1930s, allegedly along ethnical lines, which don't coincide with what Stalin did in reality...
My theory is that Iosif was drunk on Vodka, picked up a pencil and started to draw the inner borders of the Soviet Union inspired by the drawings of 5 year old children...