"Does it matter if a particular Gosho is authentic? Surely the criteria should be wisdom." -- SGI member

Teachings in question from the True Aspect of All Phenomena:

"The common mortal is the entity of the three properties, or the True Buddha, while the Buddha is the function of the three properties or a provisional Buddha."

and

"Therefore, the two Buddhas, Shakyamuni and Many Treasures, are Buddhas who are functions [of Myoho-renge-kyo]. It is Myoho-renge-kyo that is the true Buddha."

Can wisdom be separated  from honesty? It is wise to be honest. If one is not honest, one is not wise. Courage too is a part of wisdom. Cowards can never be wise. Let me give you a couple of scenarios. 

The author of the teaching in question believed this teaching to be more profound than the Daishonin's teachings but  was neither honest enough nor courageous enough to argue the merits of his teachings. What is a dishonest and/or cowardly person to do when he wishes to assert his viewpoint? Attribute it to the founder. 

The author of the teaching in question may even have thought that he was being compassionate but he had no documentary proof to make the case for his assertions. He felt that he had grasped the essence of the Daishonin's teachings and his insight would greatly benefit the masses of beings. There were three problems however: He was a junior monk at some temple and knew that no one would listen to him; the Daishonin never taught that which he perceived; and the montei (reading between the lines) argument was not an accepted standard for evaluating the Daishonin's teaching. What is a sincere, compassionate [but deluded man] to do "to benefit the people" when he knows he will not be headed? Attribute it to the author. This is known as a "pious" forgery.

The reason that these teachings are fallacious [though very attractive to the conceited people of Mappo], are several-fold: There is no concept of True Buddha versus false Buddha in Buddhism; Shakyamuni Buddha possesed the three properties as do we all. Now, if he was referring to Amida or Dainichi Buddha, a more powerful argument could be made regarding the lack of possession of the three properties but since the author is referring to Shakayamuni, this argument is without merit; these teachings contradict those found in such emminent writings as the 16th Chapter of the Lotus Sutra and the Kaimoku Sho, not to mention the entire body of the Lotus Sutra and authenticated Gosho. There is no Buddha known by the name of Namu Myoho renge kyo nor Myoho renge kyo and Namu Myoho renge kyo and Myoho renge kyo are known as the Treasure of the Law, not the Treasure of the Buddha.