To be honest Russia isn't as strongly denying it anymore. I've long thought that the DPRK should send troops both to crush fascism and the US/NATO empire and because sad as it is that war costs lives, the DPRK has no warfare experience within the last 60 years and things have changed a lot. A few brigades getting experience on modern combat with Russia could take those lessons back and save many lives if/when the occupied south attacks with the Americans.

Also AFAIK didn't occupied Korea already send a bunch of shells to Ukraine? They may not have been operating at full steam in support of them but I'm pretty sure they already chose to get involved.

Though I am still dubious that this has happened it wouldn't be that strange and wouldn't change my position that Russia has the right to do this.

Though I do find it a bit unlikely Russia would actually allow it as it would give the west a pretext to pour their own troops over the border by saying Russia did it first with an ally and all is fair. Most likely the DPRK troops that are in Russia reported by the west are just there for training and learning the latest combat lessons from Russian troops in modern warfare.

Hopefully the Russian Duma ratifies the treaty on mutual defense and cooperation soon as that would certainly deter the US somewhat from attacking Korea.