I think the difference between sycophancy and NVC (at least how I learned it) is that a sycophantic person just uncritically agrees with you, but NVC is about how to communicate disagreement, so the other person actually listen to your argument instead of adopting a reflexive defense response.
I think the problem is that telling someone they're wrong without hurting their ego is a very difficult skill to learn. And even if you're really good at it, you'll still often fail because sometimes people just don't want to be disagreed with regardless of how you phrase it. It's far easier for the AI to learn to be a sycophant instead (or on the opposite side of the spectrum, to learn to just not care about hurting people's feelings).
