The Index of PDL Documentation is not an index. It's a 'Table of Contents'. It's not alphabetised. It list only the equivalent of Chapter heading, not keywords or functions or anything vaguely resembling an index.
Much in the same vein as "Question" vs "Doubt", I wonder if this could be possibly due to the fact that in some languages, e.g. my own, which happens to be Italian, you have indice and indice analitico for table of contents and index respectively, so that a naive, uninformed translation may likely fail.