You are 100% correct in your assertion that the civil war was a culmination of much more than just moral outrage over slavery, and it's a subject worth continued study.
However, there are people who are exploiting that nuance for despicable reasons. So when you comment trying to clarify what you see as a matter of historical record, some of us see it as unhelpful because it's continuing to provide conversational cover to those who want to use that historical record in bad faith.
It's true, some slaves learned trade skills, but would you come in talking that ish if the OP was about the benefits of being enslaved?
It is a bit hard to distinguish between a bad faith arguing and someone who is being pedantic. Poe's Law may parallel this. Maybe that's what they thought?