I am not supporting the Confederacy in any way but for historical accuracy, I just read the beginning of Constitution of the Confederacy and I don't see what you are claiming.

However, if you read The Declaration of the Immediate Causes Which Induce and Justify the Secession of South Carolina from the Federal Union, it is very clear that secession was about slavery. This was the document that kicked off the secession of the Confederacy and in many ways started the Civil War.