Who is considered mixed race in Korea?

This morning I read an interesting article about the "Baby Boom of Mixed Children in Korea". Since I was expecting the article to highlight that these mixed race kids come from Korean and foreign (black, white, east indian, etc) parents I was surprised to read the parents that caused these kids to be "mixed" were other Asians of a different culture and nationality.

So I have to confess I don't understand how having a parent who's korean and another who is Thai makes a child mixed race. I'm sure I'm missing something but can't figure out what. To me black parents from two different cultures still make a "black" child so why is it different with 2 asian parents? When did asian start separating into different race classifications?!

And if having 2 asians parent makes a child mixed race then what about children like my son with only 1 asian parent? Is there a new classification for them?