And was this just a post-hoc justification, or had the western powers declared that they would retaliate if Russia involved other armies?

In any case, surely the 'punishment' should be directed at North Korea?

North Korean troops are helping Russia invade Ukraine (by freeing up Russian garrison troops to participate in their offensive).

Ergo, redress is something that helps Ukraine resist the military advantage North Korean involvement gives Russia -- e.g. being able to target Russian military targets supporting the invasion, in Russia.

The problem is at least as much Russia inviting NK as North Korea positively responding, aiding Ukraine works against all the belligerents aligned against it, NK as well as Russia, and the North Koreans in Russia are not protected by the Armistice the way North Koreans on the Korean peninsula are.