It is one reason, but not the only reason. NK and China both have borders connecting to Russia's border. This was extremely critical during WW2 as Japanese had full control of Korea (i.e. both present day of NK and SK), and Japanese already took over the northern part of China (they began to invade China in 1931). Russian and Japanese were never at ease because they were afraid the other would strike first (they did have multiple battles throughout WW2).

Russia has no big problem with China (their economy really does benefit a lot doing trading with China, although Russia is not happy China is making tons of money from defense contracts, whose technology basis came from the old Russia Federation), so to keep the American military busy they can't turn their back on NK completely. They certainly do not like NK at all, but they need NK there for now. Until NK is out of bound, China will tune down and continue pressure Kim in the back channel. But trust me... Chairman Xi, he is absolutely a tiger.

For all the superpowers, their best game is flexing muscle. They enjoy talking on stage, and drinking wines behind door.