The excerpt you quoted clearly shows the author does not believe the 12th Imam existed and that the first safeer "and his associates succeeded in preventing this confused state of affairs from deteriorating further by advocating the doctrine of occultation (al-ghayba) of the Imam, viz., that the newly born child Imam, son of al-'Askari, had gone into hiding due to adverse political conditions and that he would emerge as soon as circumstances would allow him to do so." .
The notion that the 12th Imam was an invention of the Banu Nawbakht in order to perpetuate the Imami creed is one that has been postulated time and again, both by Sunni polemicists and Orientalists alike. It ignores evidence of ahadeeth that pre-date the 11th Imam re: the number of Imams being 12, and the occultation of the last of them (al-Qa'im bi 'l Haqq) .
It simply presupposes that there was, in fact, no son born to the 11th Imam, and then uses that presupposition as a basis for the thesis that the 4 sufara worked together to promulgate the idea of an Imam in ghayba who would appear when it was safe to do so in order to stop the Imami community from disintegrating upon finding out that there was no longer a living Imam to lead them .
Yes, I do ...