The pilot flying is not expecting to hear "FEATHER" at 100 dBa. This is not what one would hear when the intended flaps are actuated and they would have, at that time, lost very little airspeed and both would focus on why there was a FEATHER announcement - the PM would still have his hand on the lever which would be a huge hint.
There didn't seem to be any stress on either pilot when the error was made - the PM was seemingly distracted, not stressed; the PM was simply not paying attention**. The flight was smooth enough, the day was nice, the weather was clear, the runway was in sight. The confusion and stress came later when they noticed the airspeed had dropped.
The problem with physical interlocks is they can prevent motion that is desired. It's also far harder to get the logic right versus the relative simplicity of adding a couple of relays, switches, and a pre-recorded message box.
**OK - I wasn't there, but wouldn't it have been acknowledged by the initial investigation if alarms and alerts had been going off when the PF asked for flaps and a terrified PM just grabbed controls at random in a panic?