It’s almost impossible to say since everyone’s definition of the term varies. To me, the safest bet is just to break it down etymologically. K-pop is short for “Korean pop” — so that’s what K-pop is.
Pop music, of course, has no real musical definition; a pop song by Billie Eilish and a pop song by Lizzo will be on two very different planes of musical stylization, but both are pop regardless. The same goes for Korean pop; anything that constitutes popular music in Korea, whether it be a ballad by Davichi, a rap song by Zico, or an idol song by TWICE, is K-pop.
The only thing I could consider K-pop not being is music that is not pop, or is not Korean. And those two qualifiers are both very loose, as noted in the conversation. Are idol groups based in Korea but comprised of non-Korean members truly “Korean music”?
This is simply how genre works and how it’s always worked. In the hip-hop world, there are still aggressive conversations about what is and what isn’t hip-hop, whether it be experimental rap records by Shabazz Palaces or Death Grips, or “mumble rap” records (which itself is undefinable as a genre). There are so many variations from genre terms that the more we study them, the less useful we find them.