leslie's guiding traditions
In the first edition of 'Scouting for Boys', it said "If a stranger makes the scout's sign to you, you should acknowledge it by making the sign back to him, and then shake hands with the LEFT HAND. If he then shows his scout's badge, or proves that he is a scout, you must treat him as a brother-scout, and help him in any way you can." So the left handshake was presented as one of the 'secret signs' of the scouts, but no particular reason was given for it being chosen at that stage, other than that practicality would demand it as the salute is given with the right hand.
So it appears that any claims about it being down to one reason or another are revisionist.