

Definitely battlefield experience and battle testing their weapons. No one wants to end up in a war and find out that their guns don’t work, their missiles are inaccurate and their air defence can’t intercept anything. It’s the same reason Iran is sending their new domestically made anti ballistic missile defence systems to Russia for use in the war. They want to test them in real combat, and improve on any flaws.
A lot of the escalation between the DPRK and ROK was because of the now impeached ROK president and the Biden administration exempting the ROK from certain arms treaties, so the situation should calm down for now. But it will heat up again, maybe very soon. So it’s always best to be prepared.
There was more evidence aside from that, even if you ignored all of that Ukrainian propaganda (which you should) there was still lots of evidence of participation by the DPRK in the war.
See my comment here