This worked for me - I was able to change it to sort by SKU by replacing “title” with “sku” in the above code snippet.
When testing this change beware the caching behavior of your browser - I was regenerating a packing slip for the same order multiple times - and it was not sorting correctly - when I moved to a different order it worked like a charm. The browser (I presume) was caching the “old” packing slip even though I was going into the order and instructing it to “print packing slip” each time. Just something to watch out for.