Restore default only works on the particular Preference setting you show. So by going to:
Preferences ==> Import/Export ==> saving books to disk
You will restore the default only for Saving books to disk. same goes for any other Restore defaults button; they exist in every single Preferences submenu (except for plugins and Sharing books by email, which are grayed out) but it only restores the default of the submenu view you have open.
Also, the replacement text I gave you for per-device templates is also the default for saving to disk and the universal send to device template; I only told you to use this instead of restore default because the only way to restore the default (that I know of) is to remove the profile from the configuration folder, which is needlessly complex.
TL;DR -- restore default will only delete the currently-viewed template, or just use
Code:
{author_sort}/{title} - {authors}
there too. Either works.