Earlier quoted context omitted.
I'm not particularly familiar with the matter, but the actions and wirittings are still subject to interpretation. Some examples might be: What was the reason for the confederacy to declare independence. What were General E Lees motivations for joining the war. What was the motivation for most soldiers (who were not slave owners) joining the war. You can find hundreds of books disputing these topics.
Once again, reasons and motivations aren't the subject of "fact checking" in the sense we're discussing. That's interpretive history, not factual stuff. And I'd be shocked beyond belief if Facebook was actually stepping in here, which is why I asked for a cite. I'll be blunt: this didn't happen. They never fact checked a post about a confederate general, and if they did it was over some kind of obviously false thing…
I would argue that this is clearly in the realm of interpretive history.
The core statement "Lee opposed both secession and slavery" is too poorly defined to have an objective answer. It is a question of what a historical figure thought and felt comes with all the nuance of a real human being.
Part of what I object to with the "fact checking" is that it often tries to adjudicate questions that are matter of opinion. Perhaps "oponion" should be the facebook flag.
Does placing the warning imply that the inverse is true and "Lee supported both secession and slavery".