The uniform electrical code published by the NFPA doesn't really say WHY a specific wire should or shouldn't work, they avoid this by merely saying it shall be approved for the application.

So I don't really sweat it - I just use UL approved (for in wall use) wire and sleep well at night. The stuff I used was CL3 rated. In new construction or jobs that will be inspected the inspector can and should refuse to pass the job until it is done to code, and that includes the correct wire.

I spent part of my misspent youth as a licensed journeyman electrician.

Charlie