Now, 64-bit Windows on the Alpha AXP would never ship. But the Alpha AXP did have the advantage of existing in physical form, so the system could finish booting before the heat death of the universe. The assumption is that most of the effort in porting Windows to the Itanium is in the 32-bit to 64-bit transition, and not in dealing with quirks of the specific 64-bit processor you are porting to.