Hal,
You say my instructors have been wrong, but that is just your opinion. There is nothing definitive in the NEC that makes it clear what Article applies.
The FPN does not give any information that applies to this discussion. For it to apply here, it would have to define signaling and communications circuits and it does not do that.
I find my source to be authoritative for this discussion. They are the people who actually install these systems.
I will research the scope statements when I have time, but probably after the holidays. I am sure that the original scope of 800 was only voice communications and will have to be convinced that has been changed by the CMP.
For what it is worth, the handbook commentary says that the data system within a building is an Article 725 installation. However that statement is just opinion, just like yours or mine.
Although information technology equipment systems are often used for or with communications systems, Article 800 does not cover wiring of this equipment. Instead, Article 645 provides requirements for wiring contained solely within an information technology equipment (computer) room. (See 645.4 for a description of the type of information technology equipment room to which Article 645 applies.) Article 725 provides requirements for wiring that extends beyond a computer room and also covers wiring of local area networks within buildings. Article 760 covers wiring requirements for fire alarm systems.
In some cases, telephone system wiring is also used for data transmission; this use is covered by Article 800.
The last part of the handbook commentary clouds the issue, but in general the authors of the handbook say that the data installation is covered by 725.