Much of the debate on the virtues of 'guessing' versus 'pure logic' flounders on inexact definitions of the two terms. I believe that Pappocom would take

to postulate a possibility and follow through the consequences

to mean a guess. Others would simply regard it as

reductio ad absurdum - or logic.

I don't quite agree with your statement:

Since, in fact, there is a unique solution, any puzzle can be solved by logic!

Even when there are multiple solutions, they are obtainable by logic, provided that the

reductio ad absurdum reasoning frowned upon by Pappocom is considered 'logic'. (You've stated that you do consider it logic). An Ariadne's thread solver, which is driven by the

reductio ad absurdum/guess procedure, is guaranteed to find all possible solutions to a Sudoku puzzle. How could the four solutions to DT Puzzle 3 have been found if not by logic? What alternative mechanism could have been employed?

I think the answer is that you define the terms 'logic' and 'guess' differently to how I do.