If you overwrite the installed files with the installed files the end result is that you've done nothing.
Whatever you're doing is unrelated to koreader.sh because you're entirely in Lua. If it's not tar because you're getting a koreader.installed.tar, then logically it's either the
download of the zsync file (which will show in debug output) or zsync itself instead.
Either way, you run the commands yourself.
./tar -whatever etc
./zsync -o ota/some-filename
http://ota.koreader.rocks etc etc
You can just print the relevant string:
https://github.com/koreader/koreader....lua#L368-L387
Whichever fails, fails. But you already know it's because you mount ota to /opt/ram/koreader/ota.