You may have to install this into a "Version 4.0" (without the quotes) subfolder in the scripts panel folder to make it work.
It seems this script does not work selectively.
The problems is a book with 30 chapters all of them threaded.
Manually it seems a serious loss of time.
I also have Text Stitch by Rorohiko that works very good but works in a massive way or manually.
The idea is to have a criteria: master page, section or paragraph style to do the unstitch.
Certainly the original version from Dave Saunders worked selectively to divide a story into two parts, the selected frame being the start of the second story, and the revised version is doing the same for me here.