That border is either mountain ranges (Altai), or empty steppes (Zabaikalsk), or boreal forests (Amur and Vladivostok). Either way -- the area lacks infrastructure and even roads, for the most part. It doesn't provide any military advantage, especially given that the Chinese side of the border is much better developed.

If anything, the area is not defensible against even a half-ass incursion effort from the south (either economical or military-driven), which they're actually in the middle of (the economical one for now) without realizing that.