This is my opinion based on my understanding of watching the ritual, and reading some other discussion with E.A. and others that is available here on the forum.
As far as I can tell there are least two major reasons for not using and Amulet of Magus or something similar.
One. The ritual was done to make contact with a particular spirit. The methods used were based on research and personal experience. In particular the method of the working was attempting to follow (to the best degree possible in the circumstances) the practices of the tradition and cosmogony of that particular spirit. This means honoring the understandings of the intent and methods of that historical tradition, and excluding things that are part of some different or non-aligned system.
Two. The intent of the rite was to make substantial contact with the spirit. Including a protection or device designed to block that spirit or to protect against it would be weakening that contact, and communicating two things. One, the operator would be admitting to themselves that they may not want the results of what they are doing to succeed, increasing their doubts about either the ritual itself, or whether they really wanted to have that contact. Second, the spirit who is subsequently contacted may then notice and respond to the devices or protection that were to be utilized against it. This could cause the spirit to either decide not to be involved, or to take whatever action was necessary to break past those protections. Either of these would ultimately reduce the efficacy of the contact.
So, in that tradition and specifically for the purpose of this rite, an Amulet of Magus would not have been appropriate since it is not part of that tradition in which the working was taking place, and it’s use would have been an admission of uncertainty by the operator, and a possible insult to the spirit in question.