If this were not so, then we would never be able to experience pleasure at all, because what we would be experiencing would be some incomplete pleasure, some mixture of pleasure and pain, which from this perspective is not pleasure at all. A "mixture" is not the same thing as a "thing in its pure form."
So this "pleasure is the absence of pain" is necessary to comprehend that it is possible to experience pleasure at all.
This sounds a lot like the argument that you can never get from point A to point B, because you can only advance half of the way at a time and the halfways keep getting smaller, ad infinitum. Can't remember the name of the argument....