I think some people on one side preferred hyperbole to facts. They also wanted foreigners not to think they were fighting to preserve slavery, which had been outlawed in nearly every Christian country at the time.
Slavery had been banned in most but certainly not all of the Western world by that time and many had only banned it comparatively recently. President Davis, with the support of the Confederate Congress, empowered his ambassador with plenipotentiary powers....meaning he could commit the CSA to a treaty to emancipate the slaves in exchange for British and French military aid. He had always maintained that the Southern states were not fighting over slavery. The US Congress similarly passed a resolution early on saying they were not fighting over slavery either.