Things to remember: the USSR got kicked out of Afghanistan, just as the US out of Vietnam. N.K. may already be aware of such things, and simply be using up spare military capacity, in the sense of clearing out the sell-by date stuff, such as older, less-fit soldiers who, otherwise, would have to be fed. Presumably, the ones in Europe will be on the Russian account regarding food and lodgings.
It would probably make for a worse situation for the Ukrainians if S.K. were to send in troops to help out. But hey, proxies can be useful, and battles on third-party territory always preferable to using one's own soil.
It's okay to have the biggest muscles, but they can't always be used without self-destruction being involved, whether physically or economically.