It is a gray area. I do not have a problem with old wiring methods if they test OK but there are other guys who do not agree.
There is a distinction between "fire alarms" and "smoke detectors" It is the smoke detector that has to be on AFCIs. Yes they do need to be battery backed up.
Nothing would surprise me. The AFCI was rammed through before Cuttler Hammer even had a working unit on the market ... and they were the people who did it. One of my neighbors was consulting on the project in the late 90s. I have often asked why CPSC did not order a recall and have CH/SqD pay to replace all of those early units that were largely ineffective. The AFCI took about 3 swings to get where it is now.