Also located on that same site is an article by the same author on making enlarged negatives using APHS film which is available from Freestyle. Although at the time that Bob wrote the earlier article he was engaged in working with PT-Pd and the procedure that he discribes was targeted at that process. That will explain his comments in the first article about his initial intentions were to use Azo as a proofing material but he soon discovered that the Azo prints were of a quality that merited mounting and framing.
From what I have heard, Azo and Pt-Pd can both be printed from the same density range negative. Your question on the contrast issue will also be answered in the article about enlarged negatives.
Additionally there are several other sites that deal with the matter of making enlarged negatives. As I recall the sites that I found were located by doing a search on platinum printing. Some of the other authors used and recommended other films then those mentioned by Bob Herbst. So the materials used can apparently vary, but consistancy seems to be the underlying thread. I hope that this directs you to the answers for your questions. Good luck.