Well, yes, they did say that, but you're reading it literally, like some lawyer reading the fine print of a municipal bond. Don't overlook the more important part that spoke of "advocat[ing] negro equality, socially and politically."
All in all, the document is like a testament to human freedom, sort of an American Magna Carta.
All in all the document is what it is. A declaration of their reasons for rebelling, their version of the Declaration of Independence. There is nothing noble or Magna Carta-ish about it. And if "a decent respect to the opinions of mankind..." required that they "...declare the causes" which impelled them to the separation then it's clear that cause was defending slavery.