This feels like the better solution to me. Not only does it encourage the user towards improving their posts, the feedback may also spill into improving how they communicate with others in their day to day lives. That would be a nice side effect 
Perhaps such feedback should only appear when a user’s topic is significantly more toxic than the rest of the community, or if their communication style matches one which is commonly misunderstood?
And your disagreement was presented with a very reasonable tone as well. Currently I see that as a refinement issue – A better AI would see that reply as: “Generally opposing something, but with a neutral tone”
In fact, GPT4 does this:
The sentiment of the text is skeptical and concerned about the reliability and ethical implications of AI-driven sentiment analysis.
The tone of the text is cautious and questioning, reflecting unease and skepticism about AI sentiment analysis.
So it’s negative, as in not in favour, rather than negative as in rude/harmful.
Looking through the responses here, I feel the more valuable and acceptable solution would be a JIT intervention in writings that are overly toxic, or contain precursors to misunderstanding before they are posted. The user still has the right to post the original content, we just do our bit to ensure they are adequately informed about the negatives their writing may create in its current form, and present an alternative which may be more productive.