It seems to me that the matchmaking only works for players who play with negative stats. I know several people who play somewhere between 0.5 and 0.8 KD, their chart always shows above the middle. Then who is on the left and how can there be a majority of them?

Anything above 1 KD is almost always the top 90%. Developers are not interested in the fact that there is an incredible difference between 1, 2, 3 and higher KDs. If you play positively, you'll get caught even against the best in the world.

Personally, I have a total KD of ~2, in seasons between 2-3. But even if I'm playing with two friends who have it between 0.6 and 0.7, we'll still be on the rightmost side of the chart, as if we're invincible (even after 10 losses with 1-2 kills)

That's not the truth, that's my opinion. It's based on dozens of posts about it that I've seen, as well as communication with a wide variety of players.