I was using the latest FireFox and its downloader, which did not produce a zip file that worked (this is the first time I have had this issue on any download). But you were right, your file link is good. I started up Google Chrome and downloaded using it and got a perfectly good file. There is something going on with the FireFox downloading, it seems.
Anyway, I installed your program. Very nice, simple and understandable interface. I selected my section of Notes, with 799 pages in it (no subpages). Before I ran the sort, I went to the OneNote folder and did a right click, copy, paste and rename of the file so that I would have a backup, in case anything went wrong with the sort). Since your program throws up a warning that the sort can't be undone, it would be nice to give the option to do a backup or suggest the user do it, just because there is so much in OneNote that can be lost.
When I ran the sort, it took a few seconds to run the sort and complete the sort of 799 pages. Perfect results!
As an external program, it is nice as a way of ordering what you have.
I encourage you to keep up on this and maybe spread the word on other forums, like the Microsoft OneNote forums, so that others can benefit from your gem of a sort solution!