I think the author has stricter interpretation of the morse code tappings shown in movie which largely is just tappings. In actual fact there are a large variet

I think the author has stricter interpretation of the morse code tappings shown in movie which largely is just tappings. In actual fact there are a large variety of customized Morse code for wall/pipe tappings. One such is using loud tap for a dash and soft one for a dot. Same goes with another variant where a tap for a dot and a quick double tap for dash. The problem with all these is the tappers need to be very well train to produce a consistent tapping rythmn which is very difficult if you're not doing it daily let alone those shown in the movie doing it for the first time or after years of not doing it. Then on listener side need to have very sharp ear with probably perfect pitch and rythmn sense which we assume had some kind of strong musical background also equally well trained in Morse code tappings on pipe/wall and in those Morse code variant being used. In short, this will turn into "perfect shot" kind of youtube series...extremely unlikely but "common" in Hollywood movie depiction.