Popular music is jazz-based. It is much less complicated than classical styles.
You seem to imply that jazz overall is much less complicated than classical, which I don't think could be farther from the truth. I'm hoping that you were not implying that, and that you were only referring to popular music being less complicated.
Plus, it's all connected. You say pop music is jazz-based (blues-based would be more accurate), but that's a huge generality. It's like saying that your wooden desk is water-based. When the wood was in tree form, water was an important ingredient, but water has very little to do with a desk. And jazz as we know it today is very much based on classical styles, so you could say that pop music is classically based as well.
I think Liween may have been a little mislead by his friends. He said this:
I asked this is because few of my friends had taken only several mths to a year music lesson at some music school for pop music and they told me that they only played around with chords on their left hand.
That sounds about right to me. If you're taking beginning lessons in playing pop music, playing chords in your left hand is what you would do. You'd play the chords in your left hand and play the melody in your right. As the lessons progressed, I imagine you would learn more interesting ways of playing tunes. When you say "knowing all the chords", it simply means knowing how to construct a chord. Once you know how to construct a c-minor chord, you can apply that to any other key.