First of all, he never said, you shouldn’t surround an enemy force. He said, surround them, but never completely. Leave a small opening for them. If you have watched Game of Thrones: Battle of the Bastards, you’ll see why this makes sense. Althought it was fictional, I’m sure good medieval European strategists would have agreed with Sun Tzu, if they had read about it. If you surround an army completely, even the most cowards will forget about escape, unite and fight you with all they’ve got. But if there is a chance to get out, even the bravest will try to squeeze through that hole and flee. Imagine if you shout: “Stop struggling! You have nowhere to go! Stay where you are and we will come to kill you all, piss in your skull, rape your women, and enslave your children!” instead. The surrounded will no doubt fight you to their last breath, and will rejoice for every bullet they managed to put inside one of you, even at the cost of ten of their lives. Sun Tsu’s book does not only focus on winning a battle, it focuses on winning a war. In the book, you’ll find plenty of sentences like, avoid physical conflict if possible, do not fight unnecessary battles, etc. He is trying to make you understand how casualties are costly, and you must avoid it.
This particular argument here is coherent with that idea. An absolutely “cornered” enemy will fight you and possibly injure and even kill your soldiers, no matter how onesidedly stronger your troops are. It will always lower your morale, and put a financial burden on your country. That doesn’t help you win a war. It was meant as an aesthetic proportion for visually experienced objects, in two or three dimensions. A melody is a pattern of music notes, parsed out over time values, which subjectively have a pleasing effect on a listening human. The intervals between the frequencies of the notes has been fairly well established in Western Music (while Eastern music has a whole different set of note relationships). How would a ratio be applied to generation of melody lines, or time values? Not able to go into detail, I’ll say there are mathematical relationships that determine pleasing intervals between notes (as well as displeasing, like the flatted fifth, the ‘Devils interval’). But wonderfully pleasing melodies are constantly being produced by creative folk, with no regard to proportions.