@BOAC
The clues for it are in the transcript.
It appears the ATC issues the horizon and go-around commands after they disappear from his (ASR-like????) radar screen due to being close to or below runway level. He also requests that they turn their landing lights on to which the captain replies that they are already on. Why would ATC do that unless he was trying to spot them visually at a totally different altitude and could not?
The key to the puzzle is the navigator stupidly reading the radio altitude. It could not possibly do anyone any good, especially with the pilot not only being occupied with trying to spot the ground but also talking to the ATC. So, they (the pilots) were either not thinking about that ravine and/or assumed that they were being given barometric altitude until the first tree sent them a different message, which would answer why they do and say very little until the first tree says "hello".
At RA 100 meters, the 2nd pilot even reassures everyone: "everything normal/all within norms" after TAWS screams 4 warnings. Soon afterwards, at RA 80 m he commands goaround.