That's not true - certainly not what the US and South Korean intelligence communities are saying. South Korea's National Intelligence Service for example stated last week that it has evidence that 1500 North Korean troops are in Ukraine. US intelligence this week says the number has risen to 3000.
Where is the proof they are inside Ukraine? Can you provide proof?
If the US had come out with this first, I’d question it. But, that’s not what happened, and I just don’t see an upside to S. Korea to fabricate the intel. Nor is S. Korea speaking in terms of “USA, please help us.” They are speaking in terms that SK must respond.