I've tried going to the folder in "Bookmark Manager" (Library) and highlighting the particular bookmarks before selecting Export HTML < Import and Backup.
You should have to use this steps.Click on the browse button to select the exported bookmarks file you saved previously. Locate the bookmarks.html file you previously created and then click the Save button.
As per my opinion the simple way to do that would be to either edit a copy of the file bookmarks.html to remove the unwanted bookmarks or copy that section to another file.