Which makes no fucking sense. How could the invasion and occupation of a sovereign world be considered an "internal" affair of Cardassian or within Cardassia's "borders?" It was a really, really dumb fucking line and a grotesque misinterpretation of the PD.
Probably based on recognized borders. Bajor was inside the recognized border of the Cardassian Empire, it didn't really matter whether it was a sovereign world or not. Just where the line was drawn.
Unfortunately, lots of people get screwed when major powers sit down and negotiate borders.