Then we would be calling BoA's participation in Anyband as a comeback.
QFT... and Big Bang has been releasing all those videos with Kim Yuna so technically they had their "comeback" a long time ago.
I wouldn't call that a comeback.
Comebacks in Korea involve pretty much an album release, but lately, singles or mini-albums are getting popular (I don't have any idea why...). Shouting Korea was one song...the other videos were promotional videos.
Does that mean that Kim Yuna had her debut in Korea because she participated in that song? no.
Big Bang hasn't come back yet. Their album is still releasing in august and it is still awaited for.
A comeback is just that, when you comeback. Whether it be from a year or two of being away or 3 months. You still were off the stage in Korea for a long period of time. New look, new songs, new techniques.