In addition to the comments already posted, they had strategic reasons to withhold support as well. While Russia doesn't pose a serious military or economic threat to RoK directly they could cause real problems. Part of the defensive advantage they have held against the North is a decade or three of technology lead. While joking about the actual effectiveness of Russian hardware is fun, large scale tech transfer to North Korea would change the strategic balance on the peninsula, RoK has pretty clearly sought to avoid that by limiting aid to Ukraine to non lethal items.

The more Russia barters with the North though, the less likely RoK will see it that the transfer isn't taking place anyways. Russia is gambling for short term help and risking a major new source of munitions and hardware for their adversaries.