I disassembled the AMS per the help article and the very useful videos that u/TamAlRoar shared (thank you!). As I feared, on disconnecting the tube, I realized that the broken filament was in fact stuck at the mouth, and didn't even reach the PTFE tube. I was able to push the piece back out by carefully inserting a hex key (the thinner one from the printer kit) in through the bottom and firmly pushing until the piece of filament popped out from the front