Here is the thing...
You can talk about "FLCH trap" and the hold mode all you want. In the end, there are three primary things I am scanning on final...airspeed, aimpoint, and centerline. By engaging automation, you are supposed to be freeing up your spare brain cells to assist in scanning those three things. Unless something else distracted the pilots, I don't see how anyone can make an excuse for what appears to be a basic lack of airmanship and allowing a heavy aircraft with revenue passengers to get that slow. In my community (military heavy pilot) if we get more than 5 knots below computed threshold speed, someone is speaking up. If for nothing else but self preservation, I can't understand how this was allowed to happen.